A photographer opens the aperture by two stops and shortens the shutter by two stops. The negative density comes out unchanged. Why is the photograph nevertheless a different photograph?

C. Exposure is conserved, but depth of field and motion blur change by two stops

Because each control carries a second job. Two stops wider throws the background further out of focus; two stops shorter freezes movement that was previously blurred. Exposure is what the trade preserves, and everything that makes the picture look the way it looks is what it changes.