What realization is Bob questioned about regarding the door?

The selected answer, "Realize," is appropriate in the context of understanding a sudden awareness or recognition about something. When someone is questioned about a realization regarding the door, it implies that they are being asked if they have come to a particular understanding or insight about it—such as its state, a change, or its importance in a particular situation.

"Remember" would not fit well, as it suggests recalling something from the past rather than having a moment of new understanding. "Understand" is related but typically denotes a deeper comprehension rather than a moment of realization. "Notice" implies simply observing something, but it does not carry the connotation of a deeper understanding or insight that 'realize' does. Thus, "Realize" effectively captures the essence of the question about Bob's awareness concerning the door.