Which of the selections below represents a recursive query?
a. a client asks a dns server to resolve a domain name.
b. a client asks another client to resolve a domain name.
c. a dns server asks a client to resolve a domain name.
d. a dns server asks another dns server to resolve a domain name

D. A DNS server asks another DNS server to resolve a domain name

Explanation:

DNS (Domain name system) queries are essentially of two types;

i. recursive address resolution

ii. iterative or non-recursive address resolution

=> In the recursive address resolution, which represents a recursive query, when a DNS server A, receives a request from a client and does not know how to resolve such request, the DNS server sends the query to another DNS server B on behalf of this client. DNS server B can either reply to the query with the actual answer or an error message. If it's an actual answer, it returns to the client. Otherwise, it queries another DNS server, say C.

=> In the iterative address resolution, once the DNS server A receives a request from the client, it tries to reply the client with the best answer it can give. If the DNS server A does not have an answer, it refers the client to another DNS server say B. In this case, the server A does not directly query the server B. Instead, it refers the client to the server B.