What would indicate that a message had been modified?

A.
The public key has been altered.

B.
The private key has been altered.

C.
The message digest values do not match.

D.
The message has been encrypted properly.

Explanation:
Hashing algorithms generate message digests to detect if modification
has taken place. The sender and receiver independently generate their own digests
and the receiver compares these values. If they are different, the receiver knows
the message had been altered in some way.