In chloroplast, the highest number of protons are found in
Lumen of thylakoids
The correct option is D Lumen of thylakoids During photosynthetic electron transport, protons (H$$^+$$) accumulate in the thylakoid space. These protons are released into thylakoid space after each split of water molecule during photooxidation and after each electron transport between PQH$$_2$$ to cytochrome-f. Increase in the number of protons in the thylakoid space results in an increase in a proton gradient.
Hence, the correct answer is 'Lumen of thylakoids'.