The correct option is: make the inflection in the main part of the compound form.
Compound nouns are nouns that are made up of two words. The word can be of different type, for example:
- noun + noun: bedroom
- noun + verb: haircut
These terms together form a different meaning.
If you have to form the plural you should distinguish the central word. For example:
appletree> appletrees
mother-in-law> mothers-in-law