A photograph of the night sky was taken with the camera shutter open

The argument is discussing a photograph of the night sky taken with a long exposure time where the normal motion of stars caused streaks, but one bright spot did not streak. The argument is trying to explain why this spot might not be streaked even if it's caused by a celestial object.

The key here is to find a reason why a celestial object could still appear as a bright spot without streaking. This reason must relate to the extended exposure time of the camera shutter.

(A) the spot was not the brightest object in the photograph
(B) the photograph contains many streaks that astronomers can identify as caused by noncelestial objects
(C) stars in the night sky do not appear to shift position relative to each other
(D) the spot could have been caused by an object that emitted a flash that lasted for only a fraction of the time that the camera shutter was open
(E) if the camera shutter had not been open for an extended period, it would have recorded substantially fewer celestial objects

The correct choice here is (D) because it provides a logical reason for why the bright spot could remain un-streaked. If the spot was caused by an object emitting a flash that lasted for only a fraction of the time that the camera shutter was open, then it would appear as a bright spot without streaking, even if the object was moving across the sky during the exposure. This explains why the bright spot is not streaked while still allowing for the possibility that it's caused by a celestial object.