The answer is b.
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).

Male Gametophyte of Angiosperms

Angiosperms are the flowering plants, which are the most advanced and successful group of plants. They are characterized by the presence of flowers, fruits, and double fertilization. The male gametophyte of angiosperms is produced in the anther of the flower.

Formation of Male Gametophyte

The male gametophyte of angiosperms is formed by the process of microsporogenesis, which takes place in the anther. The microspore mother cell undergoes meiosis to form four haploid microspores. Each microspore develops into a pollen grain.

Structure of Pollen Grain

The pollen grain is a small, spherical structure that is shed by the anther. It consists of two layers:

1. Outer Layer (Exine)

The outer layer is made up of a tough, protective material called sporopollenin. It is highly resistant to chemical and physical degradation, which allows the pollen grain to survive in a variety of environments.

2. Inner Layer (Intine)

The inner layer is made up of cellulose and pectin. It is thin and flexible, which allows the pollen tube to grow through it during fertilization.

Types of Pollen Grain

There are two types of pollen grain based on the number of cells:

1. Three-Celled Pollen Grain

The three-celled pollen grain is the most common type of pollen grain in angiosperms. It has two sperm cells and one vegetative cell. The vegetative cell produces the pollen tube, which grows through the stigma and style to reach the ovule.

2. Four-Celled Pollen Grain

The four-celled pollen grain is a rare type of pollen grain in angiosperms. It has two sperm cells, one vegetative cell, and one sterile cell. The sterile cell may play a role in pollen tube growth or it may be vestigial.

Conclusion

The male gametophyte of angiosperms is shed as a three-celled pollen grain, which consists of two sperm cells and one vegetative cell. The pollen grain is a highly specialized structure that allows for efficient fertilization of the ovule.

Tree cellled and it's endosperm(3N)