A problem that is challenging to solve is referred to as what?

The term that describes a problem that is particularly challenging to solve is "conundrum." A conundrum is often associated with riddles or puzzles that require significant thought and ingenuity to address. This term conveys the idea of complexity or intricacy, which aligns well with the notion of a problem that presents difficulty.

The other terms provided do not convey the same meaning. "Glacial" typically refers to things related to ice or having a slow movement, and is not associated with problem-solving. "Concoction" refers to a mixture of various ingredients or elements, often used in cooking or to describe a combination of ideas, but does not imply difficulty in solving a problem. "Impervious" means not allowing something to pass through or not affected by something, which also does not pertain to the concept of a challenging problem. Hence, "conundrum" is the most fitting choice for describing a difficult problem to solve.