A person behind a moving siren will interpret the frequency of the siren as being

Answer: Smaller than it actually is.

Explanation:

The person is behind the moving siren, and in this case, we assume that the siren is moving away from the man. By the Doppler effect, we know that a wave that comes from a source that is moving away will suffer a "redshift", which means that the wavelength increases a little bit, now, as you may know, the relation between the wavelength and the frequency comes by the equation:

velocity = wavelength/frequency.

If we keep the velocity constant (the velocity of the sound is the same independent of the frequency) then, if we have an increase in the wavelength, we must have a decrease in the frequency to maintain the velocity constant. From this, we can see that a redshift is also associated with a decrease in the frequency of the wave.

Then, the person behind the moving siren will interpret the frequency as being smaller than it actually is.