Which of the following types of attack does an attacker's system appear to be the server to the real client and appear to be the client to the real server?

In the on-path (man-in-the-middle) attack, the attacker is between the client and the server, and to either end, the attacker appears like the legitimate other ends. This does not describe any denial-of-service attack. A replay attack involves resending login information. Although an on-path attack can be used to perform eavesdropping, in this scenario the best answer is an on-path attack.