Which one forms the endosperm?

Secondary one forms the endosperm.
Endosperm is formed when the two sperm nuclei inside a pollen grain reach the interior of a female gametophyte.
One sperm nucleus fertilizes the egg cell, forming a zygote, while the other sperm nucleus usually fuses with the binucleate central cell, forming a primary endosperm cell. That cell created in the process of double fertilization develops into the endosperm.
It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This can make endosperm a source of nutrition in animal diets.