The purpose of this question is to determine the whole. To solve this kind of problem, we need to pay attention to "the object under investigation should actually be the data representing a certain feature of things, not the thing under investigation." When we distinguish the four concepts of whole, individual, sample and sample size, we should first find out the object to be investigated, that is, the math scores of junior high school graduates in a certain area who took the senior high school entrance examination, then find out the samples according to the data collected in this part, and finally determine the sample size according to the samples.

12000 The math scores of junior high school graduates who took part in the senior high school entrance examination are overall, the math scores of 500 candidates are a sample, and the math scores of each candidate are individual. So, C.

This topic examines the whole, sample, individual and other related knowledge points of statistical knowledge, and it is necessary to clarify its definition. Error-prone and confusing: students easily understand the meaning of the whole and the individual and make mistakes.