The purpose of a message authentication code - MAC is to verify both the source and message integrity without the need for additional processes.

A MAC algorithm, sometimes called a keyed (cryptographic) hash function (however, cryptographic hash function is only one of the possible ways to generate MACs), accepts as input a secret key and an arbitrary-length message to be authenticated, and outputs a MAC (sometimes known as a tag). The MAC value protects both a message's data integrity as well as its authenticity, by allowing verifiers (who also possess the secret key) to detect any changes to the message content.

MACs differ from digital signatures as MAC values are both generated and verified using the same secret key. This implies that the sender and receiver of a message must agree on the same key before initiating communications, as is the case with symmetric encryption. For the same reason, MACs do not provide the property of non-repudiation offered by signatures specifically in the case of a network-wide shared secret key: any user who can verify a MAC is also capable of generating MACs for other messages.

In contrast, a digital signature is generated using the private key of a key pair, which is asymmetric encryption. Since this private key is only accessible to its holder, a digital signature proves that a document was signed by none other than that holder. Thus, digital signatures do offer non-repudiation.

The following answers are incorrect:

PAM - Pluggable Authentication Module: This isn't the right answer. There is no known message authentication function called a PAM. However, a pluggable authentication module (PAM) is a mechanism to integrate multiple low-level authentication schemes and commonly used within the Linux Operating System.

NAM - Negative Acknowledgement Message: This isn't the right answer. There is no known message authentication function called a NAM. The proper term for a negative acknowledgement is NAK, it is a signal used in digital communications to ensure that data is received with a minimum of errors.

Digital Signature Certificate: This isn't right. As it is explained and contrasted in the explanations provided above.

The following reference(s) was used to create this question:

The CCCure Computer Based Tutorial for Security+, you can subscribe at http://www.cccure.tv

and

http://en.wikipedia.org/wiki/Message_authentication_code