(a) Identify, A,

(a) A is called as Stigma in above diagram. It is the top part of carpel. The function of stigma is Receiving the pollen grains from anther of stamen during the process of pollination

B is called as Pollen tube. The function of it is: When a pollen falls on Stigma, it bursts open and grows a pollen tube downward through style towards female gamete in ovary. A male gamete moves down the pollen tube.

C is the female gamete also known as Ovum. It combines with the male gamete to form zygote.

(b) Since sexual reproduction occurs due to fusion of the reproductive cells also known as sex cells. The sex cell are of two types: Male sex cells and female sex cells. In sexual reproduction, a male gamete gets combined or fused with the female gamete resulting in the formation of a new cell known as zygote which grows in a due course of time and develops into a new organism.