How Could Astronomers Detect a Large Asteroid If It Were

How could astronomers detect a large asteroid if it were heading straight for Earth?

A) The asteroid would appear as a slowly brightening starlike object where no star was previously charted, with a redshifted solar spectrum of reflected light.
B) The asteroid would appear as a short trail against the background stars on a sky-tracked long-exposure photograph, and its spectrum would show no Doppler shift.
C) The asteroid would appear as a slowly brightening point of light where no star had previously been charted, and the spectrum of sunlight reflected from it would be blueshifted by the Doppler effect.
D) The asteroid would appear as a slowly brightening and growing diffuse sphere of light where no star was charted because of light scattered from the dust and gas surrounding it, and it would show a blueshifted spectrum.