He discovered nucleus while studying the process of fertilisation under the microscope, he found the nucleus in the reproductive cells.
Robert Brown was a Scottish Botanist who was responsible for discovery of the nucleus of the cell and he is responsible for discovering the Brownian motion which is the random movement of microscopic particles.
He was studying the process of fertilisation in orchids under the microscope and while studying this he observed an opaque area which he called as the nucleus. This is how he discovered nucleus.
If we look at the options, option A is the pollen cell of orchid. Pollen cells are the reproductive cells and he was studying the process of fertilisation where he found the nucleus in pollen cells of orchids. This option is the correct one.
Option B is a Mesophyll cell of orchid leaves but he was not studying this when he found the nucleus therefore this cannot be the correct option.
Option C is the Bark of oak tree. He discovered the nucleus while studying orchid, not oaktree so this is the wrong option.
Thus, the correct answer is option A i.e., Pollen cell of orchid.
Robert Brown made his biggest discovery while he was observing the sexual organs of plants. Under the microscope, he found out that the pollen grains are moving around in a random manner. He became curious and then he observed the same movement in glass and rock particles. He concluded that this movement is caused by some phenomenon and he names it as Brownian motion and it has become his biggest discovery.
Note: When he was studying the process of fertilisation in an orchid, it is obvious that to study fertilisation one has to look for the reproductive cells and these are the pollen cells. This clarifies the answer to the given questions and you can eliminate the options accordingly.