What instrument can be used to measure voltage output from an alternator?

A multimeter is the correct choice for measuring the voltage output from an alternator because it is specifically designed to measure various electrical parameters, including voltage, current, and resistance. When testing an alternator, a multimeter can be set to the voltage setting, allowing for a direct measurement of the alternator's output voltage. This functionality is critical in determining whether the alternator is operating correctly and supplying adequate voltage to the electrical system.

In contrast, the other instruments listed do not serve the purpose of measuring electrical voltage. A fuel gauge indicates the level of fuel in a tank and is not applicable to electrical measurements. A pressure gauge measures fluid pressure, which is also unrelated to electrical voltage. A tachometer measures engine speed in revolutions per minute (RPM) and does not provide any information about voltage output. Therefore, using a multimeter is essential for properly testing the voltage output from an alternator.