If an attacker intercepts a public key and forwards his own key to the receiver, what kind of attack is this?

A.
Replay

B.
Smurf

C.
Dictionary

D.
Man-in-the-middle

Explanation:
When an attacker intercepts messages and then pretends to be one of
the parties involved in the communication; a man-in-the-middle attack has been
performed. This is what is taking place when an attacker forwards on her public key
unknown to the receiver.