What is the main difference between interim fixes and product fix packs as they relate to IBM WebSphere
Application Server?

A.
Interim fixes typically contain new features or functionality ahead of future product fix packs.

B.
Product fix packs are a combination of interim fixes and internal fixes to make patching easier.

C.
Product fix packs are generally reserved for new features or functionality whereas interim fixes are provided
only to fix known issues.

D.
Product fix packs bring WebSphere Application Server to a new product level whereas interim fixes provide
corrective service for specific known problems.

Explanation:
Product fix packs contain bundled service to bring WebSphere Application Server up to a new product level.
Interim fixes provide corrective service for specific known problems.
https://www.ibm.com/support/knowledgecenter/en/SS7K4U_8.5.5/
com.ibm.websphere.installation.zseries.doc/ae/tins_install_fixes_z.html