Final answer:
A collection of intermediary compromised systems for a DDoS attack is referred to as a 'Botnet'. Botnets are used to overload a system with traffic to disrupt service. It's different from phishing, spoofing, or a firewall.
Step-by-step explanation:
The term that refers to a collection of intermediary compromised systems for a DDoS attack is a) Botnet. A botnet is a network of private computers infected with malicious software and controlled as a group without the owners' knowledge, e.g., to send spam messages or to participate in a distributed denial-of-service attack (DDoS).
During a DDoS attack, these compromised systems, which are also known as 'zombies', are used by attackers to flood the target with an overwhelming amount of traffic, leading to service disruption. In contrast to botnets, phishing is a technique used to deceive individuals into providing sensitive information, spoofing is the act of disguising communication from an unknown source as being from a known, trusted source, and a firewall is a network security system designed to prevent unauthorized access to or from a private network.