The correct option is C exine of pollen wall
The microsporangium of anther is surrounded by four wall layers – the epidermis, endothecium, middle layers and the tapetum.
Pollen grains develop inside microsporangium (pollen sac)
Pollen grain has a two layered wall.
The outer hard layer is called exine. It is made up of sporopollenin which is a highly resistant organic material. It can withstand high temperature, strong alkali and acids. It cannot be degraded by any enzyme known.
The inner thin layer is called intine and is made up of cellulose and pectin.
Hence sporopollenin occurs in the exine, the outer layer of pollen grain.