You have a Linux system routing 3 networks through 3 separate NICs and are having trouble with your IP forwarding.

What file would you check to ensure that IP forwarding is enabled?

A.
/etc/defaultrouter

B.
/proc/net/tcp

C.
/proc/sys/net/ipv4/ip_forward

D.
/var/log/messages

Explanation/Reference:
From http://techgurulive.com/2008/09/15/how-to-enable-ip-forwarding-in-linux-2:

To check for current status of IP forwarding on IPv4 IP class
# cat /proc/sys/net/ipv4/ip_forward
If IP forwarding is disabled, a value of 0 would be displayed and if IP forwarding is enabled, linux should be displaying a numerical value of 1.