Maybe "Which question is solved by E?" or "How many questions are solved by E?" - another way would be "Which question has E as its correct answer? // Which question is answered by E?" or "How many questions are answered by E?"

I personally like the solved wording.

Also another vote for clearer colors about possibility state - for example, the cells are marked green if that specific answer is internally consistent, but it can still be an incorrect solution overall, which means you did not get the correct answer whatsoever.

I think people (myself included) have been trained to think green = good, meaning that answer is correct and shouldn't be moved, you found that one, now work on the others - so an intermediary color as mentioned, like yellow, meaning 'yep that's plausible but not necessarily right' would be a good fit there.