Because the weight of 500 students is taken for analysis, this weight is the sample, excluding A and B, and because the number of individuals in a sample is called sample size, the sample size does not need to be taken as a unit. Individual refers to every object in the crowd, and C refers to the weight of 500 students. The weight of these 500 students is not the whole, but the sample, so C is excluded. So you should choose D.
This is my understanding, not necessarily correct. Because I did a similar question, which didn't mention the concept of sample size, but ABC was wrong, leaving D.
I hope it helps you ~ ~ _