The phrase "challenging questions" refers to difficult or complex questions that require critical thinking or problem-solving skills to answer. Full definition
Now however I see it as using the wonderful gift of humor to struggle with one of the more challenging questions of the Christian faith. (nakedpastor.com)
We need people who aren't afraid to ask challenging questions about sacred cows. (redeeminggod.com)
The first page breaks the questions down into clear steps while the second page includes more challenging questions with less support. (tes.com)