Which of the following steps causes a proton gradient to develop in the chloroplast ?
 a) Splitting water
b) Activity of PQ and cytochrome  $b_{6}F$ complex
c) NADP reductase

Three reasons for the development of a proton gradient within the lumen of thylakoids is as follows:

Splitting of the water molecule on the inner side of the membrane, the protons or hydrogen ions get accumulated within the lumen of the thylakoid

Movement of electrons across the photosystem via PQ leads to accumulation of protons within the lumen .

The utilization of protons in stroma by NADP reductase to form NADPH leads to decrease in the number of  protons in stroma.