21) D


All body cells such as red blood cells and skin cells have double the amount of DNA as gametes. 
Zygotes also have double the DNA as gametes. 
The easiest one to spot is from the ‘0’ column. Only an enucleated egg cell has ‘0’ DNA. 
Since egg cell is a gamete, it will have ‘1’ DNA. Since nerve cell is a body cell, it will have ‘2’ DNA. 


So, D is the answer.