A zygote is formed by the fusion of
Both A and B
Heterogametes are a pair of gametes that differ in form, size, or behavior and occur typically as larger non-motile female gamete (ovum) and smaller motile male gamete (sperm).
Isogametes are gametes that have the same size and structure as the one with which it unites.
The fusion of both types of gametes leads to the formation of a zygote.