In research on the need for achievement, individual differences are usually measured: A. by observing subjects' actual behavior in competitive situations. B. by interviewing subjects about their achievement needs. C. with the Thematic Apperception Test. D. with a polygraph.

Go through each option and determine if it is a possible method for measuring individual differences in the need for achievement. A. Observing subjects' actual behavior in competitive situations could be a possible method to measure the need for achievement, but it may not be the most accurate or commonly used method in this field. B. Interviewing subjects about their achievement needs might provide some insights, but self-report measures can be biased and unreliable. C. The Thematic Apperception Test (TAT) is a well-known projective psychological test used to evaluate a person's need for achievement. D. A polygraph is a lie detector that measures various physiological responses, but it is not typically used to measure individual differences in the need for achievement.

Based on the analysis of each option, the Thematic Apperception Test (TAT) is the correct answer. This test is widely known and used in psychological research to measure individual differences in the need for achievement. So the correct answer is: C. with the Thematic Apperception Test.