Endosperm tissue is unique to flowering plants, or angiosperms. Made up mostly of starch, it surrounds a seed and nourishes it.

the endosperm in monocot plants and endosperm and cotyledons in dicot plants.

An example of a seed that has liquid endosperm would be a coconut. While it does contain cellular endosperm, the coconut milk would be considered liquid endosperm.

endosperm haustoria is reported in many families for eg. Cucurbitaceae.it is a vermiform appendage present at the chalazal end of the endosperm. it may be free nuclear or cellular.

No, triploid endosperm is in angiosperms only. Gymnosperm means "Naked Seed."