Question

how does endosperm of angiosperm become triploid?

Answer 1

Endosperm is formed when the two male nuclei  inside a pollen  grain reach the interior of a female gametophyte (sometimes called the embryo sac). One male nucleus (n)fertilizes the egg cell, forming a zygote, while the other male nucleus usually fuses with the binucleate central cell(2n), forming a primary endosperm(n+2n=3n) cell (its nucleus is often called thetriple fusion nucleus). That cell created in the process of double fertilisation  develops into the endosperm.