Which of the following types of attacks occurs when an attacker successfully inserts an intermediary software or program between two communicating hosts?

A Denial-of-Service (DoS) attack aims to make a service unavailable to legitimate users, not to intercept and alter communication between two parties.

A dictionary attack is a method of trying to discover passwords by systematically entering words from a list of common passwords, not by intercepting communications.

A Man-in-the-Middle (MITM) attack is precisely characterized by an attacker interjecting themselves into an ongoing communication between two hosts, allowing them to intercept, read, and potentially modify messages before forwarding them. This makes both parties believe they are directly communicating with each other, unaware of the intermediary.

A password guessing attack is a broad term for attempts to determine a password, which can include dictionary attacks but does not describe the insertion of an intermediary between communicating hosts.