Final answer:
DNS amplification attacks exploit recursive servers by sending queries with a spoofed IP, which results in the server sending large responses to the intended target, thus overwhelming it with traffic.
Step-by-step explanation:
In a DNS amplification attack, the type of server that an attacker sends requests to is b) Recursive servers. DNS (Domain Name System) amplification attacks are a form of Distributed Denial of Service (DDoS) attack, where the attacker exploits vulnerabilities in the DNS server's recursive lookup feature. These recursive servers are responsible for resolving domain names by making a series of requests to other DNS servers until the IP address is found. The attacker sends a large number of queries with a spoofed source IP address (usually the target's IP) to open DNS recursive servers. These servers then respond with much larger replies, amplifying the attack's bandwidth and overwhelming the target with traffic. The reason recursive servers are targeted is because they are designed to accept queries from any client and can be tricked into sending large responses to the spoofed IP address.