Which word best describes a clear and readily understandable concept?

The word "limpid" perfectly captures the idea of a concept that is clear and readily understandable. Derived from Latin, "limpid" suggests clarity and transparency, much like limpid water that allows one to see clearly through it. When applied to ideas or concepts, it conveys that they are easy to grasp and free from confusion or obscurity.

In contrast, the other words present different qualities. "Opaque" describes something that is not transparent or difficult to understand. "Ambiguous" refers to a lack of clarity due to having multiple meanings, which can confuse rather than clarify. "Vague" denotes a general lack of precision or detail, leading to uncertainty. Therefore, "limpid" stands out as the most fitting choice to describe a clear and readily understandable concept.