The correct option is C Mitchell
The chemiosmotic hypothesis of ATP synthesis in chloroplasts is based on proton gradient. The chemiosmotic hypothesis was explained by P. Mitchell in 1961. He recieved noble prize for it in 1978. ATP synthesis is linked to the development of a proton gradient across the membrane of the thylakoid of chloroplast and proton accumulation is towards the inside of the membrane i.e. in the lumen.