Which of the following is considered a control instrument?

The tachometer is considered a control instrument because it directly indicates the engine's power output relative to its maximum allowable rpm. In flight operations, particularly during instrument flying, pilots need to maintain engine performance within specific limits to ensure safe and efficient operation of the aircraft. By monitoring the tachometer, pilots can make real-time adjustments to throttle settings, which is crucial for controlling the aircraft's performance during various phases of flight.

The other instruments listed serve different purposes. The altimeter provides information about altitude, which is essential for vertical positioning but does not assist in controlling engine power. The VSI (Vertical Speed Indicator) indicates the rate of climb or descent, helping pilots understand whether they are gaining or losing altitude but not directly contributing to engine power management. The airspeed indicator measures the aircraft's speed through the air, which is important for maintaining safe flight characteristics, but again, it is not a control instrument focused on engine performance.