Which of the following is given as an example of a "preventative check" on population growth?

In his historical Essay on the Principle of Population, Robert Malthus hypothesized that poverty and famine would become a global epidemic if populations continued to go unregulated. Malthus based his theory on the idea that plants and animals always produce more offspring than can survive. He had a point there, right? Maybe, but he also believed that humans, namely the lower class, were no different, and that checks needed to be in place in order to prevent the destruction of the human race. What do you think, extreme? Well, a few hundred years ago, many people didn’t think it was so far-fetched. According to Malthus, in a balanced population the population growth was controlled by "positive checks" such as disease, and "preventative checks" such as the postponement of marriage.
    As a political economist, Malthus was concerned about the direction society seemed to be heading towards in the 19th-century England. According to Malthus, living conditions were steadily declining as a result of a high birthrate, an inadequate supply of resources, and the irresponsible working class. This concern was what inspired him to write his essay. Malthus suggested that population might be controlled if the lower class took it upon themselves to only have as many children as they could financially support. He also felt that if the lower class just got a taste of luxury they might be more likely to start families later, and therefore have fewer children. While Malthus’ controversial paper made him a celebrity, many people, especially the working class, deplored his pessimism and went as far as to call him a "prophet of doom." Charles Darwin read Malthus’ essay and was inspired to develop his own theory of Natural Selection. Darwin considered overpopulation to be a necessity, in that with more offspring than food, siblings were forced to become more competitive. From this came Darwin’s theory on the survival of the fittest.
    Back to Malthus, though. We know his hypothesis was wrong, because population growth has not been exponential, and, after all, we’re all here today talking about him. So where did he go wrong? Well, first of all, Malthus failed to foresee the changes that would happen in Western society. In fact, his theories completely lost their validity by the mid-nineteenth century because he didn’t anticipate the Industrial Revolution. With new technology, agriculture was no longer the driving force of the economy, and farming became much easier. There was a boost in agricultural production, and in the Western world today we actually produce more food than our population needs. Speaking of hungry, did I just hear some stomachs growling? OK. I know, it’s break time...just a few more things and I’ll let you go. So, as I was saying, Malthus made a number of important oversights. He also forgot to take into account that although the aging population would begin to live longer, they would eventually pull out of the labor market, thus increasing the need for a steady birthrate. Rather than Malthus’ predicted population explosion, population growths in the Western world have greatly declined, namely since the 1960’s. In addition, economists today suggest that there is an internal population check after all. As families get richer, they become healthier and seem to have fewer children.
    Another thing that Malthus predicted in his essay was a loss of biodiversity. Malthus and many others since him linked a larger world population with a loss of habitat and thus an extinction of many species. The theory that a 90% loss of habitat would lead to a 50% reduction of the species has proved to be vastly exaggerated. The forests of Puerto Rico are a great example of this. In the past 400 years approximately 99% of the main forests have been cleared. Pretty crazy, huh? Anyhow, my point is, less than 10% of the species is believed to be extinct as a result. Even here at home, the forests of the Eastern states have been devastated in the past two centuries, and though many species have been reduced, there are very few cases of bird and animal extinction. Malthus’ other threat about pollution has also failed to materialize in the Western world. Studies today show that a society that becomes richer also becomes more concerned with the environment. In other words, they can afford to care for it. Take London, England for example. Some say the air quality is twice as clean there now than it was a few hundred years ago.
    With new technology, agriculture was no longer the driving force of the economy, and farming became much easier. There was a boost in agricultural production, and in the Western world today we actually produce more food than our population needs. Speaking of hungry, did I just hear some stomachs growling? OK. I know, it’s break time ... just a few more things and I’ll let you go.
    What does the professor imply when he says this:
    Speaking of hungry, did I just hear some stomachs growling? OK. I know, it’s break time ... just a few more things and I’ll let you go.