Innermost microsporangial wall layer that nourishes pollen grains is:

Explanation

The microsporangial wall comprises four layers: Epidermis, Endothecium, Middle layers and Tapetum. The epidermis is followed by a single layer of endothecium, 2-3 middle layers and a single layer of tapetum. Epidermis functions as a protective covering.Endothecium develops fibrous thickenings at maturity, which are hygroscopic in nature and helps in anther dehiscence.Middle layers are involved in the storage of reserve food material during early stages of microspores development which later get mobilized to the developing microspores.Tapetum is the innermost layer of the anther wall and surrounds the sporogenous tissue completely. It provides nutrition to the developing microspores which form the pollen grains.Hence, the innermost microsporangial wall that nourishes pollen grains is tapetum. So, the correct answer is B.