C. The main purpose of DDoS attack is to make the target server unable to provide service normally, which is one of the network attacks that are most difficult to defend. The most accurate description of a DDoS (Distributed Denial of Service) attack is that its primary purpose is to overwhelm and disrupt the target server or network's ability to provide services normally. DDoS attacks aim to flood the target with a massive volume of traffic or requests, causing server resources to become exhausted and rendering the service inaccessible to legitimate users. Defending against DDoS attacks can be challenging because of their scale and distributed nature.

C. The main purpose of DDoS attack is to make the target server unable to provide service normally, which is one of the network attacks that are most difficult to defend. A Distributed Denial of Service (DDoS) attack is a malicious attempt to disrupt the normal traffic of a target server, service, or network by overwhelming the target or its surrounding infrastructure with a surge of Internet traffic.

https://www.cloudflare.com/learning/ddos/what-is-a-ddos-attack/