An instrument that is most likely used to measure very high resistance values is a(n):

The instrument used to measure very high resistance values is a megohmmeter, which is specifically designed for this purpose. Megohmmeters can measure resistance in the range of megohms, making them suitable for testing insulation resistance of electrical systems, cables, and other components where high resistance is expected. This capability is particularly important in applications such as pipeline corrosion control, where determining the insulation integrity and resistance to electrical currents can help assess the risk of corrosion.

While a voltmeter measures voltage and an ammeter measures current, neither is intended for resistance measurement. An ohmmeter measures resistance but is typically limited to lower resistance values compared to a megohmmeter, thus making it less suitable for high resistance applications. The design of a megohmmeter allows it to provide accurate measurements even in high resistance scenarios, making it the preferred tool for professionals in fields that require detailed analysis of electrical resistance.