The correct option is C Both A and R are true and R is not the correct explanation of A
A developed male gametophyte i.e. pollen grain has two cells - the vegetative cell with abundant food reserve and the generative cell that floats in the cytoplasm of the vegetative cell.
In 60% of angiosperms, pollen grains shed from the anther after this stage (2-celled stage).
In the remaining species, the generative cell divides (mitosis) to form two male gametes after which the pollen grain sheds (3-celled stage).
Hence, assertion and reason are true but, reason is not the correct explanation of the assertion.