Which of the following best describes a conundrum?

A conundrum refers to a difficult problem or a puzzling situation that often presents a dilemma or challenge without an obvious solution. This term is commonly associated with riddles or problems that require creative thinking to find an answer. The essence of a conundrum lies in its complexity and the level of difficulty it poses to those trying to resolve it.

The other options represent different concepts: a steep cliff refers to a geographical feature, a mass of ice indicates a physical object related to glaciers, and mixing substances describes a process in chemistry or cooking. None of these options relate to the notion of a challenging problem, making the first choice the most accurate description of a conundrum.