For optimal night flying operations, the best night vision is achieved when:

The correct answer focuses on the role of the rod cells in the eyes, which are primarily responsible for vision in low-light conditions. When flying at night, optimal vision is critical for safety, and this is achieved when the rod cells, which are sensitive to dim light, have had sufficient time to adjust to darkness. This adaptation process typically takes about 30 minutes.

Upon transitioning from a well-lit environment to darkness, the eyes require time to adjust, allowing the rods to become more sensitive to light. This heightened sensitivity enables a pilot to detect objects and nuances in the night environment more effectively.

The other options suggest various timeframes for visual adaptation that do not align with the physiological processes of the eyes. Pupil dilation is quick and occurs within minutes, while cones, which are responsible for color vision and function best in bright light, adapt far quicker than rods, making them less relevant for low-light scenarios. Full adaptation, while contextually relevant, is best associated with the time it takes for rod cells to fully adjust, which is approximately 30 minutes, emphasizing the significance of option B in the context of optimal night flying operations.