What do you call a system where users need to remember only one username and password, and be authenticated for multiple services?

What do you call a system where users need to remember only one username and password,
and be authenticated for multiple services?

A.
Simple Sign-on

B.
Unique Sign-on

C.
Single Sign-on

D.
Digital Certificate

Explanation:
Single sign-on (SSO) is a specialized form of software authentication that enables a
user to authenticate once and gain access to the resources of multiple software systems.