Generation of proton gradients across membranes occurs during

The correct option is A - both photosynthesis and respiration.

Chemiosmosis is the formation of a proton gradient which is necessary to drive ATP synthesis and occurs both in mitochondrion and chloroplast. In the mitochondrion, a proton gradient is formed across the inner membrane while in the chloroplast, a proton gradient is formed across the thylakoid membrane. The gradient is broken down due to the movement of protons across the membrane through ATP synthase.