How many root server clusters are there in the DNS system?

The DNS (Domain Name System) uses a system of root servers to help resolve internet domain names into IP addresses. These servers are critical for internet functionality, as they form the backbone of the DNS hierarchy. While there are 13 sets of root server clusters, each of these servers can operate in conjunction with many instances and replicas that are distributed worldwide, allowing for high availability and redundancy.

The reason this answer is correct stems from the structure of the root name server system. Each of the 13 root servers is identified by a letter (A through M), and although they are physically distributed and can have many copies or instances across numerous locations globally, they are essentially based on 13 core root server nodes. This design allows for efficient DNS resolution without overloading any single server and ensures that the DNS service remains resilient to potential disruptions.