Final answer:
The apparent frequency of the unreflected sound waves remains the same at 400 Hz when a tuning fork moving away from an observer since there is no relative motion along the line of sight.
Step-by-step explanation:
The apparent frequency of the unreflected sound waves coming directly to the observer as a tuning fork of frequency 400 Hz is moved away from them remains unchanged at 400 Hz. This is because the frequency of the waves emitted by the source does not depend on the relative movement between the source and the observer if they are moving in the same direction. The apparent frequency change is only observed when there is relative motion along the line connecting the source and the observer, which is described by the Doppler Effect. However, since the question specifies that the tuning fork is moving directly away from the observer and not towards them, the unreflected sound waves will still have their original frequency when they reach the observer.