What feature is commonly associated with the term 'opaque'?

The term 'opaque' is often used to describe materials or substances that do not allow light to pass through, making them impenetrable. In a broader context, it can also refer to ideas or concepts that are unclear or difficult to understand. This association aligns with "impenetrable and unclear," emphasizing that something described as opaque lacks clarity and transparency, whether in a physical or metaphorical sense.

The other choices suggest characteristics that are contrary to the nature of opacity. For instance, clarity and understanding imply transparency, which directly opposes the idea of being opaque. Similarly, translucence and lucidity suggest a degree of light penetration and clarity that opaque materials inherently lack. Brightness and transparency also highlight features of clarity and openness, which are easily distinguished from the impenetrable qualities of opaqueness. Thus, the characteristics of being impenetrable and unclear align perfectly with the definition of opaque.