Question

Once double fertilization occurs in plants, the zygote is __________ and the endosperm is __________.

Answer 1

Double fertilization happens when one sperm cell wires with the egg to create a zygote, and the other sperm cell wires with the two polar cores to make the endosperm. 
To answer the above:

Once double fertilization occurs in plants, the zygote is diploid and the endosperm is triploid.