When a sound source is moving toward the observer, the observer will hear a higher pitch. this is due to

a. a decrease in the amplitude of the sound wave.
b. a decrease in the frequency of the sound wave.
c. an increase in the frequency of the sound wave.
d. an increase in the amplitude of the sound wave.

Number one a and b are automatically out if you decrease the pitch or frequency you wont hear it louder it does not add up ok?
number 2 the answer would be C the higher the frequency the higher the pitch the lower the frequency the lower the pitch this just comes back to common sense. for example you cant decrease the frequency and than expect a louder pitch its like saying I am going to smash the key on this piano with all my strength and it will make a small very low pitch sound you get what I mean so your answer would indeed be C.