A、Summer. B、Fall. C、Winter. D、Spring. D信息明示题，文章提到Spring appears to be the best period of the year for thinking，所以D正确

If you are like most people, your intelligence varies from season to season. You are probably a lot sharper in the spring than you are at any other time of the year.
    A noted scientist, Ellsworth Huntington, concluded from other men’s work and his own among people that different climate and temperature have a definite effect on our mental abilities.
    He found that cool weather is’ much more favorable for creative thinking than is summer heat. This does not mean that all people are less intelligent in summer than they are during the rest of the year. it does mean, however, that the mental abilities of large numbers of people tend to be lowest in summer.
Spring appears to be the best period of the year for thinking. One reason may be that in the spring man’s mental abilities are affected by the same factors that bring about great changes in all nature.
    Fall is the next best season, then winter. As for summer, it seems to be a good time to take a long vacation from thinking.