Question

Why did Hershey and Chase radioactively label viral DNA with 32P and viral protein with 35S? (Figure 13.5)

Answer 1

Basically you can answer the question if you know the meaning of 32P and of 35S.

32P means Phosporus-32 (32 is the mass) and 35S means the Sulfur isotope with mass of 35 (it's the most radioactive of all the sulfur isotopes btw).

So we can see that the names have to do with Sulfur and Phosphorus - therefore, the best answer is the following one:

C. Because DNA contains phosphorus and proteins contain sulfur

Answer 2

C.

DNA molecules consist of phosphorus, while proteins contain sulfur.
These properties resulted in phosphorus being used to mark DNA and sulfur being used to mark proteins.