In over 60 percent of angiosperms, pollen grains are shed at

The correct option is D Two-celled stage

Pollen grain
Pollen grain is a haploid structure that represents the male gametophyte because the male gametes are formed from them.
Shedding stage of pollen grains
The microspores of anther divide mitotically to produce two daughter cells, a small generative cell and a large vegetative cell. Pollen grains in over 60% of the angiosperms (mostly dicots) are shed at this 2 celled stage.

In some species, pollen grains are shed at the 3 celled stage. This happens when the generative nucleus further divides mitotically to form two male gametes.

Final answer:
(a) Two-celled stage