What does a firewall check to prevent particular ports and applications from getting packets into an
organization?

A.
Transport layer port numbers and application layer headers

B.
Presentation layer headers and the session layer port numbers

C.
Network layer headers and the session layer port numbers

D.
Application layer port numbers and the transport layer headers

Explanation:
Newer firewalls can filter traffic based on many packet attributes like source IP address, source port,
destination IP address or transport layer port, destination service like WWW or FTP. They can filter based on
protocols, TTL values, netblock of originator, of the source, and many other attributes.
Application layer firewalls are responsible for filtering at 3, 4, 5, 7 layer. Because they analyze the application
layer headers, most firewall control and filtering is performed actually in the software.
https://en.wikipedia.org/wiki/Firewall_(computing)#Network_layer_or_packet_filters
http://howdoesinternetwork.com/2012/application-layer-firewalls