Endosperm of flowering plants develops from

Flowering plants are Angiosperms. Angiosperms have a unique phenomenon of "Double Fertilisation" in which, after the release of two male gametes inside the embryo sac by the pollen tube, one male gamete i.e. the first male gamete which is too near to egg fuses with egg nucleus and this phenomenon is known as fertilisation or syngamy. During syngamy, a zygote is formed. The other male gamete i.e. the second male gamete migrates towards the secondary nucleus (formed from the fusion of two polar nuclei) and fuses with it. This phenomenon is known as triple fusion and results in the formation of a triploid primary endosperm mother cell which forms triploid endosperm. Thus, Endosperm is formed from a triploid nucleus.

So, the correct answer is C.