The answers include A (.dll) and B (.exe). The options D,E and F are not correct at all. I believe the option C should have been .scr which is a PE file to make it three.
The basic WildFire service is included as part of the Palo Alto Networks next generation firewall and does not require a WildFire subscription. With the basic WildFire service, the firewall can forward portable executable (PE) files for WildFire analysis, and can retrieve WildFire signatures only with antivirus and/or Threat Prevention updates which are made available every 24-48 hours.
https://www.paloaltonetworks.com/documentation/81/wildfire/wf_admin/wildfire-overview/wildfire-subscription