Which tool would be most useful for checking the voltage output of a vehicle's electrical system?

A multimeter is the most useful tool for checking the voltage output of a vehicle's electrical system because it is specifically designed to measure electrical properties, including voltage, current, and resistance. When testing a vehicle's electrical system, a multimeter allows technicians to evaluate various components, diagnose issues, and ensure that the electrical system is functioning within the manufacturer's specifications.

Utilizing a multimeter can help identify problems like battery issues, alternator output, and voltage drops across connections. This versatility makes it an essential tool for troubleshooting and maintaining the health of a vehicle's electrical systems.

The other options do not serve the purpose of checking voltage. An odometer is used to measure the distance traveled by the vehicle, a tire pressure gauge measures the air pressure in tires, and a fuel pressure tester measures the pressure in the fuel system. None of these tools relate to electrical measurements, highlighting why they are not suitable for checking voltage output.