An important element of Darwin’s second postulate about variation is that

Question

A. Species vary 

B. The environment varies 

C. Individuals vary 

D. Reproduction varies 

E. Variation can be limited 

Answer 1

Darwin's four postulates state that 1. The individuals within a species vary from one another. 2. Some of the variations seen within the individuals of a species are passed down to the next generation. 3. An excess of offspring are produced (many will die), ensuring that an adequate number survive to pass on their genetic material. 4) Reproduction and survival are not random. Only those with favorable traits live long enough to reproduce and pass on their traits.

So, from your list of answers, D seems like the most likely option.