Answer:
B) Postzygotic
Step-by-step explanation:
If the offspring resulting from the crossing of the two fly species is not viable, it means that although the hybrids were produced, they do not have the biological characteristics to survive and therefore, that there is a postzygotic barrier (it occurs after the zygote has formed).
A postzygotic barrier is condition that prevents hybrids from surviving (dying) or reproducing (infertility).