I bought an exterior prehung fiberglass door that was already primed. I want to paint it now. Do I use water based paint? I don’t know what it was primed with at the factory.

You have to scuff it a little bit with sanding sponge then use exterior paint, it can be latex

Just use exterior paint

Have to use oil base paint water base paint will wash off.

Don't matter because it is a exterior door!

I don't think it matters as long as it is an exterior paint. Although a oil based paint would probably last longer.