When the mass of the object placed on the plate increases, the slide moves down to access the circuit resistance becomes larger, the total resistance in the circuit becomes larger,
According to Ohm's law can be obtained, the current in the circuit becomes smaller, the voltage at the ends of the resistor R becomes smaller,
Because of the total voltage in a series circuit
Since the total voltage in a series circuit is equal to the sum of the partial voltages,
therefore, the voltage across R0 becomes larger, i.e., the voltmeter becomes larger.
So choose A.
The voltage across R0 is greater.