Endosperm in the angiosperms is formed by

The correct option is C

Triple fusion and double fertilisation

Fertilisation in female gametophyte of angiosperms involves two important events called as syngamy and triple fusion respectively. In syngamy, one of the sperm released into the emryo sac fuses with egg and forms the zygote which futher develops into the embryo. In triple fusion, the other sperm fuses with the central cell in the embryo sac to form primary endosperm nucleus which further develops into the endosperm (a nutritive tissue which nourishes the developing zygote into embryo. These more or less simultaneous events, that is syngamy and triple fusion, are collectively termed double fertilisation, Hence the event of triple fusion in double fertilisation leads to formation of endosperm.