Which of the following events take place after double fertilisation?

The correct option is D The PEN (primary endosperm nucleus) develops into endosperm
After pollination, the compatible pollen grain germinates on the stigma to form the pollen tube. The pollen tube enters the embryo sac which then proceeds to release the two sperms in the megagametophyte.
Of the two male gametes released, one fertilises the egg cell to form a diploid zygote, the process called syngamy. The other fuses with the two polar nuclei of the large central cell of the embryo sac to form a triploid nucleus (primary endosperm nucleus). This is called triple fusion. The PEN (primary endosperm nucleus) then develops into endosperm. Hence, it is a post-fertilisation event.