How is the term "impenetrable" best defined?

The term "impenetrable" is best defined as "impossible to understand." This definition captures the essence of the word, which connotes a sense of obscurity or lack of clarity that prevents comprehension. When something is described as impenetrable, it suggests that it is dense or complex to the point that it cannot be easily understood or deciphered.

In contrast, the other options suggest clarity and ease of understanding, which are the opposite of what "impenetrable" signifies. Thus, the correct choice aligns perfectly with the concept of something being resistant to understanding or interpretation.