The correct option is B Microspore mother cells.
The
sporogeneous tissue which may divide in various planes and finally separate
from each other during the development of microsporangium is known as
microspore mother cell. Meiosis can be best observed in microspore mother
cells. The formation of haploid microspores from diploid microspore mother cell
inside pollen sac takes place through meiosis. This process is known as
microsporogenesis. The haploid microspore formed from a single microspore mother
cell (pollen mother cell) are arranged in the form of four cells called
microspore tetrad (haploid).