Which file is used to determine which tty devices the root user is allowed to log in to?
a. /var/securetty
b. /home/securetty
c. /usr/securetty
d. /etc/securetty?

The  file that is used to determine which tty devices the root user is allowed to log in to is: /etc/securetty
etc/securetty is consulted by pam_securetty module in order to decide from which virtual terminals root is allowed to login from. You can limit the access to root account by disabling root logins at the console by editing the /etc/securetty file. 

Css3 is the latest evolution of the cascading style sheets language and aims at extending css2.1. it brings a lot of new features and additions, like rounded corners, shadows, gradients, transitions or animations, as well as new layouts like multi-columns, flexible box or grid layouts.credits to medium for making this answer credits to google for copying this answercredits to me for putting this answer on your question