Final answer:
Diffie-Hellman is a key exchange protocol used in secure internet communications.
Step-by-step explanation:
The correct answer is a. A key exchange protocol.
Diffie-Hellman is a cryptographic protocol that allows two parties to establish a shared secret key over an insecure communication channel. It is commonly used in secure internet communications, such as HTTPS, VPNs, and secure email.
The Diffie-Hellman protocol provides a way for two parties to generate a shared secret key without actually sharing the key over the network. This key can then be used for symmetric encryption to secure the communication between the parties.