Embryo develops from the ___(a)_____at the ___(b)______ of the embryo sac.

The correct option is B (a) - zygote, (b) - micropylar end
The two ends of the embryo sac are the micropylar end and the chalazal end.
The egg cell is located at the micropylar end. The male gamete fuses with the egg cell to form zygote.
Hence, the zygote is formed at the micropylar end. Thus the development of the embryo from the zygote also occurs at this end.

Primary endosperm nucleus (PEN) is formed as a result of triple fusion. PEN forms the endosperm in the primary endosperm cell of the embryo sac.