(a) embryo sac
Correct Option (d)
Reason : Megasporangium is equivalent to an ovule. An ovule generally has a single embryo sac formed from a megaspore through reduction division. It is a small structure attached to the placenta by means of a stalk called funicle.
Each has one or two protective envelopes called integuments. Nucellus is a mass of cells enclosed with in the integuments. ovule forms Megaspore Mother Cell (MMC) by meiosis which further forms megaspore.
Megaspore nucleus forms embryo sac. Fruits develop from the ovary of flower, other floral parts degenerate and fall off.