What is an endosperm mother cell?

The correct option is B Central cell and its two polar nuclei
Endosperm mother cell refers to the cell which gives rise to the endosperm. One of the male gametes, during the process of fertilization combines with the polar nuclei in the central cell to form the primary endosperm nucleus (PEN) which develops into the endosperm. Hence, the central cell and its two polar nuclei are referred to as the endosperm mother cell.

A mature embryo sac consists of eight cells. The whole of the mature embryo sac doesn’t give rise to the endosperm and hence it cannot be called the endosperm mother cell.

Ovary is one of the three main parts of the pistil of the plants. It contains the embryo sac.

Antipodal cells are found at the chalazal end of the mature embryo sac. These cells degenerate after fertilization.