In a chloroplast the highest number of protons are found in:

Step-by-step solution

During photophosphorylation process, PS-II helps in the photolysis of water molecules, and the PS-II are located towards the lumen of the thylakoid. The photolysis yields number of protons. For the photolysis of the water several cations and anions are needed, like Magnesium, Manganese and Chloride. Thus, the lumen of thylakoid has a maximum number of protons.