Variation occurs in the progeny formed through sexual reproduction because in this process both the germ cells are equally involved. Reproduction involves fusion of male and female gametes. So, each time when zygote is formed, it is formed by combining of variants.
(b) (i) The part marked 'A' is Pollen grains.
(ii) 'A' reach part 'B' through the process of pollination. Bis stigma of the carpel.
(iii) 'C' is pollen tube. It helps in reaching of male gamete to the egg ovule.
(iv) 'D' is egg. After fertilization, the egg divides several times and form an embryo within the ovule.