Option d is the answer because Pollen grains produced  from meiosis of microspore mother cells

Introduction:
Pollen grains play a crucial role in the reproduction of seed plants. They are responsible for the transfer of male gametes from the anther to the stigma, where fertilization takes place. Understanding the source of pollen grains is essential in comprehending the process of plant reproduction.
The source of pollen grains:
Pollen grains are produced from the male reproductive organ of a flower, known as the anther. The anther is a part of the stamen, which is the male reproductive structure of a flower. Therefore, the correct answer to the given question is none of these (D) because none of the options provided (stigma, ovary, style) are involved in the production of pollen grains.
Process of pollen grain production:
The production of pollen grains occurs within the anther through a process called microsporogenesis. This process involves the development and differentiation of microspore mother cells, which eventually give rise to pollen grains. The microspore mother cells undergo meiosis, resulting in the formation of four haploid microspores. Each microspore then develops into a mature pollen grain, containing the male gametes required for fertilization.
Role of pollen grains:
Once the pollen grains are produced, they are released from the anther and are typically transported by various agents such as wind, water, insects, or animals to the female reproductive structure of a flower, which is the stigma. The pollen grains then germinate on the stigma and form a pollen tube, allowing the male gametes to travel down to the ovary for fertilization.
Conclusion:
In summary, pollen grains are produced from the anther, which is part of the stamen in a flower. They play a crucial role in the transfer of male gametes and the process of plant reproduction.