Answer:
b. thick wall.
Explanation:
Endospore is a structure formed by some bacteria, that help them survive unfavourable conditions (e.g. lack of nutrients). This survival strategy of some bacteria (usually Gram+ bacteria) help them stay dormant for a while, until stressful conditions stop. A thick wall composed of many layers (exosporium, spore coat, spore cortex, core wall) of the endospore is what provides resistance to different stressful chemical and physical factors such as UV radiation, temperature, chemical damage etc.
