Assertion A is incorrect because the preferred voltmeter for measuring the potential difference across a resistance of 600 ohm is actually the voltmeter with a higher resistance, not a lower resistance. The reason for this is that when a voltmeter is connected in parallel with the resistance being measured, it will draw current away from the resistance, reducing the potential difference across it. A higher resistance voltmeter will draw less current and therefore have a smaller effect on the potential difference being measured.

In this case, the voltmeter with resistance 4000 ohm would be preferred because it would draw less current than the voltmeter with resistance 1000 ohm, leading to a more accurate measurement of the potential difference across the resistance of 600 ohm.

Therefore, assertion A is incorrect because it states that the voltmeter with resistance 1000 ohm is preferred, when in reality the voltmeter with resistance 4000 ohm is preferred.

Reason R states that a voltmeter with higher resistance will draw smaller current than a voltmeter with lower resistance. This is correct because Ohm's law states that the current through a resistor is proportional to the voltage across it and inversely proportional to the resistance. Thus, a voltmeter with higher resistance will draw less current, making it a better choice for measuring the potential difference across another resistance.