The rules of formal grammar say that you should use "which" because you're choosing from within a finite group (singers), but no one would misunderstand what you were asking if you used "what." In fact, I'd bet that more native speakers in the US would use "what."

Both are fine! But "what" is more casual/informal and "which" is more formal/bookish.

The sentence should be "Which singer has the most beautiful voice?"

Use "which" if you want to emphasize that a selection is involved. Use "what" to emphasize that you want to know the answer. Neither is always best.

I don’t know which is the right option, can someone answer me? please