Answer: Higher pitch
Step-by-step explanation:
From Doppler's effect we understand that when a source of sound approaches a listener, the apparent frequency perceived by the listener is higher than the actual frequency of the source. And the pitch is directly related to frequency. Higher the frequency, higher is the pitch.
Thus, when the ambulance with the siren on approaches the dog in it front, the dog would hear a higher pitch sound since the waves are pushed together in front of the car which means number waves per unit time increases which means frequency and hence pitch of the sound increases.