Which instrument is designed to measure the rate of climb or descent of an aircraft?

The vertical velocity indicator is specifically designed to measure the rate of climb or descent of an aircraft. This instrument, often referred to as a variometer or VVI, provides pilots with essential information about whether the aircraft is gaining or losing altitude and at what rate. It typically displays this information in feet per minute or meters per second, enabling pilots to assess their ascent or descent performance accurately.

The other instruments listed serve different purposes. The airspeed indicator measures the speed of the aircraft relative to the air, which helps pilots determine their flight performance and ensure safe operation. The tachometer indicates the engine's RPM (revolutions per minute), providing data on engine performance rather than altitude changes. A clock is used to keep track of time and does not provide any specific information about altitude changes. Thus, the vertical velocity indicator is the correct choice for monitoring climb or descent rates.