How many characters of the username can long?

To create a user account, keep in mind that the username is at most __ characters long.

A.
6

B.
8

C.
12

D.
18

Explanation/Reference:
From the Linux Admin FAQ:

6.17 What is the maximum username length, and is there way to extend it?
No. Username length is not configurable. Under libc-5, the utmp and wtmp files only allow 8 characters for a username.

Under libc-6, this is increased to 32 characters. You can use usernames longer than this limit. However, the utmp/wtmp entries will be truncated, and so won’t correspond to a valid username. This doesn’t matter for most things.