Which of the following is a restriction for IFR flight?

The correct answer is based on the premise that IFR, or Instrument Flight Rules, specifically require pilots to navigate using instruments rather than relying on visual cues or external references. This is essential in conditions where visibility is poor, such as in clouds or heavy precipitation, and ensures that pilots can safely operate their aircraft in a controlled manner regardless of weather conditions.

When flying under IFR, pilots must be trained and certified to use aircraft instruments for navigation, altitude, and attitude control. This restriction is designed to enhance safety by mandating that pilots rely on instruments that provide accurate data about the aircraft's position and environment, especially when flying in airspace where visual references are compromised.

In contrast, other options reference conditions that either imply reliance on visual navigation or restrict operating solely under certain visual conditions, which are not consistent with IFR operations. Instrument navigation is crucial for maintaining safety and efficiency in varied weather scenarios and high-traffic airspace. Therefore, the mandatory use of instruments for navigation solidifies this answer as the key restriction for IFR flight.