There are some processes that occur in both photosynthesis and cellular respiration. Which process is common to both cellular respiration and photosynthesis?
I. Chemiosmosis
II. Electron transport chain
III. Production of water

The organelle involved in cellular respiration is mitochondrion and the one involved in photosynthesis is chloroplast.

I. Chemiosmosis is the formation of proton gradient which is necessary to drive ATP synthesis and occurs both in mitochondrion and chloroplast. In mitochondrion, a proton gradient is formed across the inner membrane while in chloroplast, a proton gradient is formed across the thylakoid membrane. 

Hence, chemiosmosis occurs both in cellular respiration and photosynthesis.

II. Electron Transport Chain or ETC consists of various enzymes and coenzymes and is involved in ATP production. In mitochondrion, ETC is present in the inner mitochondrial membrane while in chloroplast, ETC is present in the thylakoid membrane.

Hence, ETC is present in both cellular respiration and photosynthesis.

III. Production of water is associated only with photosynthesis and not with cellular respiration.

So, the correct answer is 'I and II only'.