A, the math scores of 2000 candidates are a sample of the whole, so A does not meet the meaning of the question;

B, each candidate's score is individual, so B does not meet the meaning of the question;

C, the math scores of these 50 thousand candidates are all overall, so C meets the meaning of the question;

D, sampling survey is taken in statistics, so D does not meet the meaning of the question;

So choose: C.