At the ip header is exposed and so filtering or firewalls can be applied to ip addresses. select one:
a. layer 1
b. layer 2
c. layer 3
d. layer 4

The IP packet header is what tells an IP-based host what to do with the packet that was received. 
The TCP/IP model describes the protocols used by the Internet. The third layer is the Network layer and in the case of the Internet (the Internet Protocol IP). The Internet layer is responsible for addressing, packaging, and routing functions.
At layer 3 the IP header is exposed and so filtering or firewalls can be applied to IP addresses.