I think kenney1321 was asking about the parallel of "What is the effect," and the word "what," Polish co, since the translation uses jaki which translates as "which." This is confusing to English-speakers because we use "what" here. If you translate literally Co jest efekt, it doesn't work.
I'd say that "which" translates to "który", as it suggests a finite set of answers (option A, option B, option C).
I'd also say, but that is only an opinion/interpretation, that "what" can be a bit... adjectival someone. And it's this 'adjectival' what that translates to "który".
"Co jest efektem?" would be a valid sentence, but anyway a less natural one than "Jaki jest efekt?"
I am writing to kenney1321 actually but couldnt answer him. I think that you should think jaki like something like "what kind of". Generaly co goes to nouns and jaki to adjectives. Example. 1. - co to jest? to jest samochód. 2. Jaki samochód? Czerwony/syzbki/stary (etc) samochód.