Which of the following is NOT a required instrument for VFR Day?

In the context of VFR (Visual Flight Rules) operations during the day, the required instruments for helicopters are defined by regulations that prioritize basic flight capabilities and safety. The correct answer identifies an instrument that is not deemed essential for VFR day flight.

A gyroscopic heading indicator, while useful for navigation and situational awareness, is not a requirement for VFR day operations in helicopters. The primary focus of VFR day requirements is on instruments that provide fundamental information necessary for safe flight, such as the airspeed indicator for assessing speed relative to the stall warning and the oil temperature gauge for monitoring engine performance.

In contrast, the oil temperature gauge, airspeed indicator, and manifold pressure gauge are considered essential because they provide crucial information about the aircraft's engine performance and operational limits. This ensures that the pilot can maintain control and optimize performance under various flight conditions. The omission of the gyroscopic heading indicator from the list of required instruments reflects its non-critical role in ensuring compliance with basic operational safety during VFR day flying.