What is pool.ntp.org?

A.
A deprecated feature for maintaining system time in the Linux kernel

B.
A website which provides binary and source packages for the OpenNTPD project

C.
A virtual cluster of various timeservers

D.
A community website used to discuss the localization of Linux

Explanation/Reference:
From http://www.pool.ntp.org/en:
The pool.ntp.org project is a big virtual cluster of timeservers providing reliable easy to use NTP service for millions of clients.