Correct option (b) three-celled pollen grain
Explanation
In angiosperms, pollen grains are generally shed from the anther at 2- celled stage, i.e. one generative cell and one vegetative cell. But, in some angiosperms, the generative cell divides prior to the dehiscence of anther and shed at 3-celled stage, i.e. one vegetative cell and two male gametes. Double fertilisation was discovered by Nawaschin (1898) in Fritillaria and Lillium. It was confirmed by Guignard (1899).