(a) Double fertilization means the fusion of one male gamete with egg cell and other male gamete with the polar nuclei, this is known as triple fusion.
(b)When male gamete fuses with egg cell it forms diploid zygote (2n) and when the other male gamete fuses with the polar nuclei (2n) it forms a triploid endosperm (3n).
© Endosperm is a female gametophytic tissue which provides nutrition to the embryo.
(d) It was first seen by Nawaschin (1998).