"two rooms are connected by a doorway. you are in one room and your friend is in the other room. explain why you can hear your friend with out seeing them, but you can only see them if they are in a direct line with you and the door. remember, the subject of this section is diffraction and interference."

You can  hear your friend because of diffraction of sound waves.

Explanation:

Through diffraction of sound waves, the sound waves can bend around the door spaces and spread out through any small opening on the door to reach you in the other room yet the other person is not in a direct line with you.The wavelength of sound waves in this case are longer enough to bend around the obstacle(door and the indirect path) and spread out to reach your room for you to hear your friend without directly being in line with the door for you to see them.

i guess it’s a and b

c. magnitude and direction