A, b, keep the switch closed, the potential difference between the two ends of the capacitor is equal to the electromotive force of the power supply, so the potential difference between the two ends of the capacitor is unchanged, and the pointer opening angle is unchanged, so AB is wrong;

C, disconnect the key, the charging capacity of the capacitor remains unchanged. If AB is closer, d will decrease. According to C=εs4πkd, the capacitance will increase. According to U=QC, the potential difference will decrease and the pointer opening angle will decrease, so c is wrong.

D, disconnect the electric key, and the charging capacity of the capacitor remains unchanged. If AB is separated, d will increase. According to C=εs4πkd, the capacitance will decrease. According to U=QC, the potential difference will increase and the pointer angle will also increase, so D is correct.

Therefore, choose: d.