Which of the following is a function of the vehicle’s alternator?

The primary function of a vehicle's alternator is to charge the battery while the engine is running. It converts mechanical energy from the engine into electrical energy, which replenishes the battery's charge and powers the vehicle's electrical systems during operation. This ensures that the battery remains charged and can supply the necessary power for starting the engine and operating electrical components when the vehicle is idle. The alternator plays a crucial role in maintaining the vehicle's electrical system, ensuring that everything from lights to power accessories functions correctly.

The other options, while related to the vehicle's operation, do not accurately describe the function of the alternator. The starter motor is responsible for starting the engine, fuel efficiency monitoring is typically managed by the vehicle’s engine control unit and related systems, and measuring battery voltage involves instruments associated with gauging and diagnostic tools, rather than the alternator itself. Thus, the correct choice emphasizes the alternator's essential role in charging the battery.