The fault-tolerant nature of the DNS ensures that the service continues functioning even if one name server goes down.
d) the service continues functioning even if one name server goes down.
Step-by-step explanation:
In internet DNS do the important role, It translate domain name such as (google.co.in, bing.com etc..) to valid IP address, Which is understandable format for computer to get the domain page access. It also play an important role to access emails for specific domain.
DNS fault-tolerant is which try to fetch service continues functioning where named server goes down. System continuously pinging the named server the server or services goes down.
As example If a person ring the doorbell continually for locked house.