What part requires a key to be turned for current to flow to the starter solenoid?

The ignition switch is a crucial component in the starting system of a vehicle. It serves the primary function of allowing electrical current to flow from the battery to various systems in the car, including the starter solenoid. When the key is turned in the ignition switch, it completes the electrical circuit, enabling the flow of current that activates the starter solenoid. This then engages the starter motor, which cranks the engine to start the vehicle.

The other options do not directly control the flow of current to the starter solenoid. The starter relay, while important for amplifying the current to the starter, does not require a key to operate—it is activated by signals from the ignition switch. The battery terminal is simply a connection point for electrical current and does not require a key. The fuel pump switch plays a role in providing fuel to the engine but is not involved in the starter activation process.