Refer to figures.

Blind Spot

Visual information from the retina is transported to the brain through the optic nerve. This is the point at the back of the eye where all the nerve fibres meet and form the nerve. This section of the retina does not contain photosensitive cells. Light that falls on the retina in this spot cannot be processed into visual impressions, so objects that are projected onto this area cannot be seen.

This blind spot is most problematic when vision from the other eye is partially or fully obscured, as that extra eye is usually what would fill in the blank section of vision, hence the reason we do not notice our blind spot in everyday life. If a pilot is looking in one direction with one eye obscured, there is a good chance that they would not see an aircraft on a constant relative bearing (stationary in the windshield) if it happens to lie within the blind spot of the working eye, meaning that the pilot may not be aware of the problem until it has come so close that its depiction is larger than the blind spot – which will most likely be too late to avoid a collision. This could be avoided by using a constant scan, to make sure the blind spot area is not always in the same place.

Therefore, in this question, there needs to be both an obscured view (so both eyes cannot see the approaching aircraft), and the aircraft must be in a constant place in the windshield, for the worst case scenario to be realised. The pilot must also have fixed vision (not scanning), as scanning would likely reveal the oncoming aircraft. This leads us to the correct answer.