A, because the size of the input voltage and the turns ratio of the transformer remain unchanged, the output voltage of the transformer remains unchanged, so A is correct;

B. When S is turned on, the total resistance of the circuit decreases and the total current increases, so the voltage consumed on the resistor R increases. Because the output voltage is constant, the voltage and current of bulb L 1 decrease, so B is wrong;

When C.S is on, the total resistance of the circuit decreases and the total current increases, while the turns ratio of the transformer remains unchanged, so the current in the primary coil increases, so C is wrong;

D, the transformer output voltage is constant, the output current becomes larger, so the output power becomes larger, and the transformer input power is equal to the output power, so the transformer input power becomes larger, so D is wrong.

So choose a.