Which one of the following, if embedded within the ciphertext, will decrease the likelihood of a
message being replayed?

A.
Stop bit

B.
Checksum

C.
Timestamp

D.
Digital signature

Explanation:
CBC is the CBC mode of some block cipher, HMAC is a keyed message digest, MD
is a plain message digest, and timestamp is to protect against replay attacks. From the OpenSSL
project http://www.mail-archive.com/openssl-users@openssl.org/msg23576.html