Eliminate (I) "all bookstores in California," because we're choosing a SAMPLE, not an entire population of stores.
Keeping this in mind, we'd not choose "Internet bookstores" unless it's clear that we're sampling these stores, not looking at all of them.
This leaves only "randomly selected children's bookstores." Even now I'm not comfortable, because this choice focuses specifically on children's stores (instead of on bookstores in general).
