Nosewheel steering can only be accomplished by the Captain using the tiller wheel, meaning the aircraft can only be taxied straight ahead if the Captain becomes incapacitated.

Nosewheel steering on the CRJ550 is not limited solely to the Captain using the tiller. In fact, the aircraft is designed so that both the Captain and the First Officer can control the nosewheel steering using the tiller. This dual capability allows for greater flexibility and safety during ground operations, enabling the aircraft to be maneuvered effectively even if one pilot becomes incapacitated.

If the Captain were incapacitated, the First Officer could use the tiller to steer the aircraft, including taxiing or turning as needed. Therefore, it is not correct to state that the aircraft can only be taxied straight ahead in the event of the Captain's incapacitation, as the co-pilot also has the ability to manage nosewheel steering. This design enhances operational safety by ensuring that ground maneuvering is not entirely dependent on one pilot.

The other choices pertain to scenarios in which steering may or may not be accessible or necessary, but the crux of the matter is that the inclusion of both pilots in this function is significant for effective ground handling.