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.