Which configuration file does sudo read when determining if a user is permitted to run applications with root privileges?

A.
/etc/groups

B.
/etc/passwd

C.
/etc/sudoers

D.
/etc/sudo.conf

Explanation/Reference:
From the man pages:
sudo determines who is an authorized user by consulting the file @sysconfdir@/sudoers. (typically /etc/sudoers)