Which occurs after double fertilization?

The pollen grain is transferred on to the surface of stigma during pollination. The pollen grain then geminates and pollen tube comes out. Pollen tube penetrates the stigmatic tissue and travels through the style to reach the ovary. It then enters the embryo sac and releases its contents in the embryo sac. Two male gametes are released by the pollen tube, one of which fuses with the egg cell and produce a diploid zygote (syngamy) and the other male gamete fuses with the diploid secondary nucleus to form a triploid primary endosperm mother cell (triple fusion). This simultaneous occurrence of syngamy and triple fusion is called double fertilisation. Thus, triple fusion results in the formation of a triploid primary endosperm nucleus or PEN which then develops into endosperm.

Hence, primary endosperm nucleus develops into endosperm after double fertilisation.

So, the correct answer is D.