Chemiosmosis is a process by which ATP (adenosine triphosphate) is produced in the cell. It relies on a concentration gradient of protons (H+ ions) across a membrane. The proton gradient is created by a proton pump. As protons flow back across the membrane, down their concentration gradient, they pass through a protein complex called ATP synthase, which uses the energy of the proton flow to produce ATP.

So, the correct answer is :

Option D : Membrane, proton pump, proton gradient, ATP synthase