A. alternating current abcd generated in rectangular coil When the tap P of the primary coil slides upward, the number of turns of the primary coil increases. According to the voltage is proportional to the number of turns, the output voltage decreases, so the bulb will dim, so A is correct;

B. When the capacitance c of the capacitor becomes larger, the capacitive reactance decreases, so the main current increases and the light bulb becomes brighter, so B is wrong;

C When the coil is in the position shown in the figure and in the neutral surface position, the instantaneous value of the induced electromotive force is zero; But the voltmeter measures the effective value, not zero, so c is wrong;

D, if the rotation angular velocity of the coil becomes 2 Ω, the maximum value is increased to 2 times according to the maximum electromotive force formula Em = NBSω Ω; The effective value E=Em2 also becomes 2 times; The voltmeter reading becomes 2 times, so d is correct;

Therefore: ad.