The correct answer is option A: Tapetum helps in the dehiscence of anther.

Explanation:
Tapetum is a specialized layer of cells present in the anther of a flower. It plays several important roles in the development and maturation of pollen grains. However, it does not directly contribute to the dehiscence of the anther.

Here is a detailed explanation of each statement:

a) Tapetum helps in the dehiscence of anther.
The tapetum is responsible for providing nourishment to developing pollen grains by secreting various nutrients and enzymes. It also helps in the formation of the pollen wall and the deposition of pollen coat substances. However, the dehiscence of the anther, which is the process of opening and releasing the mature pollen grains, is primarily controlled by the structural and mechanical properties of the anther wall, including the arrangement and elasticity of its cell layers. Therefore, statement A is not true.

b) Exine of pollen grains is made up of sporopollenin.
The exine is the outer layer of the pollen grain wall. It is composed mainly of a chemically resistant polymer called sporopollenin. Sporopollenin is highly durable and protects the pollen grain from environmental stresses such as desiccation, UV radiation, and microbial attack. Therefore, statement B is true.

c) Pollen grains of many species cause severe allergies.
Pollen grains are microscopic structures produced by the male reproductive organs of flowering plants. They are released into the air and can be inhaled by humans and animals. In some individuals, exposure to certain types of pollen can trigger allergic reactions, leading to symptoms such as sneezing, itching, watery eyes, and respiratory difficulties. Therefore, statement C is true.

d) Stored pollen in liquid nitrogen can be used in crop breeding programs.
Pollen preservation techniques, such as storing pollen in liquid nitrogen at ultra-low temperatures, can help maintain the genetic diversity of plant species and facilitate crop breeding programs. By preserving pollen samples, breeders can store genetic material from desirable traits for future use. Therefore, statement D is true.

In conclusion, the statement that is not true is option A: Tapetum helps in the dehiscence of anther.