Electrical converter needed for Croatia

Are you asking about a converter to convert 220 volts to 110 volt or an adapter for the wall plug? An E/F plug is an adapter and not a converter. Do not confuse the two. Results can be very unpleasant.

You probably just need a plug adapter, and Croatia uses "C" as does most of Europe:
https://www.worldstandards.eu/electricity/spread-plug-types-map

I am looking for what to use for a wall plug. I am from Michigan going to Croatia. I have E/F. Now I have ordered C; not sure which to take. It is mainly for charging phone and iPad.

Here's what the link I posted above says:
European outlets going forward will be grounded E/F.
Plugs C,E,F will all fit. Cell phones, etc. generally don't have grounded plugs, and I have used C adapters with no issues. But both C and E/F will meet your needs.