Because the voltage of the power supply is unchanged, the indication of the voltmeter remains unchanged;

When the slider P of the sliding rheostat moves to the right, the resistance of the sliding rheostat connected to the circuit becomes large.

According to ohm's law, the current passing through the sliding rheostat becomes smaller, that is, the indication of ammeter A 1 becomes smaller;

Because the voltage across the parallel circuit is constant, and the resistance of the constant resistor R 1 is constant, the current flowing through R 1 is constant according to Ohm's law.

Also, because the main current of the parallel circuit is equal to the sum of the currents of each branch, the current in the main circuit becomes smaller, that is, the indication of the ammeter A2 becomes smaller;

The sum of the readings of ammeter A 1 and ammeter A2 becomes smaller;

The ratio of voltmeter V to ammeter A2 becomes larger.

So choose a.