At the time of anther dehiscence
A. Middle layers develop fibrous thickening
B. Epidermis degenerates
C. Endothecium develops fibrous thickening
D. Endothecium degenerates

Hint: The outer layers of walls remain thin and endothecium has hygroscopic nature which means that it attracts water from its surroundings, through absorption or adsorptions. Example: honey, methanol, ethanol, etc. Thus, there is a difference in layers of anther.

Complete answer:
Anther which is male reproductive part of plants. This consists of four layers that is, epidermis, endothecium, middle layer and tapetum. Anther dehiscence is the final process of development of anther in which it releases the pollen grains from its surface. This process includes pollen differentiation, floral development and flower opening.
Thus, among these layers, the endothecium layer is the layer which helps in the anther dehiscence. The cells of endothecium are radially elongated and develop into fibrous thickening which will arise from inner walls of the anther when it matures. These fibrous materials are made up of alpha cellulose and traces of lignin content.
The inner walls of anther are unable to stretch or develop as the anther matures because of the fibrous thickenings but the outer layers are thin and they can expand as the anther matures.
Due to this uneven and differential expansion of outer and inner walls or layers and hygroscopic nature of endothelial cells, the endothecium layer helps in the anther dehiscence at the maturity.

Thus, we can say that at the time of anther dehiscence, fibrous thickenings are formed in endothecium. Thus, option ‘C’ is the correct option for this question.

Note: Due to the different nature of outer and inner layers of anther and hygroscopic nature of the endothelial layer of anther, only the endothecium layer develops into fibrous thickening and helps in anther dehiscence. And pollen from anther comes out of the anther.