A is the best answer. Lines 45-51 indicate that substances that function as morphogenetic determinants are inactive in the unfertilized egg and that when the egg is fertilized, they “become active and, presumably, govern the behavior of the genes they interact with.” In the fourth paragraph, we learn that these substances exert their control over the fate of the cell by directing “the synthesis of histones.” Because these histones cannot be synthesized until the substances that function as morphogenetic determinants become active, and because these substances do not become active until the egg is fertilized, it can be inferred that the synthesis of the histones is dependent on the fertilization of the egg.