Final answer:
To complete the proof, use the given information to establish a chain of equal angles.
Step-by-step explanation:
To complete the proof, we need to make a chain of equal angles using the given information. Since M5 is equal to M6, we can say:
M3 = M5 = M6 = M4
So, option a) is the correct answer: M3 = M5 = M6 = M4