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It may be inferred that a moving observer could perceive the waves emitted by a moving object
We know that if there is relative motion between the two objects then the Doppler Effect occurs. However, if both the object and the source are moving at exactly the same velocity in exactly the same direction, there would be no relative movement and the object would appear to be stationary. Therefore, a moving observer could perceive the waves from a moving object as if the object were stationary (under these exact conditions).
A moving observer can use the Doppler Effect to calculate the velocity of a moving object, not of the waves that object emits.
The second sentence tells us that the Doppler Effect is contingent on an object that is traveling slower than the wave it emits. However, this does not mean that, should an object travel faster than its waves, an observer will not be able to perceive those waves, only that the observer will perceive them without the effects of the Doppler Effect.
This answer choice would be true if it stated that the observer was traveling away from from the object faster than the speed of the waves (an observer can't perceive waves unless those waves reach him or her). However, the answer choice merely says different than, which could mean that the objects are traveling slower or faster.
We know that the Doppler Effect still occurs through various media and that the object's velocity and direction can still be calculated as long as the effect of the medium is known. Therefore, interference from a medium does not necessarily prevent waves from being perceived by an observer.