For a given hash function H, to prevent substitution of a message M1 for
a message M2, it is necessary that:

A.
H(M1) = H(M2)

B.
H(M1) <> H(M2)

C.
H(M1) < H(M2)

D.
H(M1) > H(M2)

Explanation:
The correct answer is H(M1) <> H(M2).
If H(M1) = H(M2) then the message digests of both M1 and M2 would be equal, and one would not
know that one message was substituted for the other. The other
answers are distracters.