Answer
(a) greater than
(b) III
Work Step by Step
(a) We know that when the horn is approaching the observer at rest, the frequency of the sound heard by the observer will be greater than the frequency of sound heard when the observer moves towards the source, which is at rest.
(b) We know that the best explanation is option (III) -- that is, a moving source causes the wave crests to "bunch up", leading to a higher frequency than for a moving observer.
