Final answer:
In angiosperms, the mature male gametophyte is made up of three cells - one generative cell and one tube cell in each pollen grain.
Step-by-step explanation:
In angiosperms, the mature male gametophyte consists of three cells. Therefore, the correct answer to the given question is b. 3. Each pollen grain, which is the male gametophyte, contains two cells: one generative cell that divides to form two sperm cells, and one tube cell that develops into the pollen tube. This structure enables the process of double fertilization, where one sperm fertilizes the egg, resulting in a diploid zygote, and the other sperm fertilizes the central cell to form the triploid endosperm.