A, the resistance in the series circuit plays the role of voltage sharing. The greater the resistance, the greater the shared voltage, the greater the indication of the oil meter and the smaller the current in the circuit. Therefore, the higher the oil level, the higher the float rises, the higher the slider moves, the greater the resistance of the sliding rheostat connected to the circuit, the greater the shared voltage, and the greater the indication of the oil meter.

R and R are connected in series in the circuit.

C, the higher the oil level, the float rises and the slider moves up. The greater the resistance of the sliding rheostat connected to the circuit, the smaller the current according to Ohm's law.

D. The oil gauge is connected in parallel with the sliding rheostat, and the oil gauge is a voltmeter.

So choose a.