Which instrument is not required for day VFR flight?

For day VFR (Visual Flight Rules) flight, specific instruments are required to ensure safe operation. The oil temperature indicator is not among the instruments mandated for day VFR flight. Its primary purpose is to provide information regarding the engine's operating temperature, which is crucial for engine health but not essential for regulatory compliance in visual flight conditions.

In contrast, the compass, altimeter, and fuel quantity indicator are all required for day VFR operations. The compass aids in navigation by providing directional guidance, the altimeter is necessary for maintaining appropriate altitude, and the fuel quantity indicator is vital for ensuring that there is sufficient fuel for the flight. Without these instruments, a pilot may face challenges in safe navigation and altitude management. Hence, the presence of the oil temperature indicator is not a regulatory requirement for day VFR flight, making it the correct answer.