Which tool is essential for measuring voltage in a circuit?

The essential tool for measuring voltage in a circuit is a multimeter. A multimeter is a versatile instrument that can measure various electrical parameters, including voltage, current, and resistance. It typically has different settings to accommodate both AC and DC voltage measurements, making it an indispensable tool for electricians and technicians when assessing electrical systems.

In contrast, other tools mentioned serve very different purposes. A wrench is used for gripping and turning nuts and bolts; it's not equipped to measure electrical properties. A soldering iron is used to join electronic components by melting solder; while it's crucial for assembly and repairs, it does not measure voltage. A hammer is a basic tool used for driving nails or breaking objects, and it has no function in electrical measurements. Thus, the multimeter stands out as the correct choice due to its specific role in electrical diagnostics.