The correct answer is B: clear. The sentence explicitly states that the explanation was "easy to understand," which directly points to its quality of being unambiguous and readily comprehensible. A clear explanation is one that is expressed in a way that is easy to perceive, understand, or interpret. It means the information is presented logically, without unnecessary jargon or complexity, allowing the audience to grasp the concepts effortlessly. The direct correlation between being 'clear' and 'easy to understand' makes this the most appropriate choice.
Let's consider why the other options are incorrect. A: confusing means difficult to understand or follow, which is the exact opposite of "easy to understand." If an explanation were confusing, it would certainly not be easy to comprehend. C: vague implies a lack of clarity or precision, making it hard to grasp the exact meaning, again contradicting "easy to understand." D: difficult is a general term indicating something that requires a lot of effort or skill to do or understand, which is also contrary to the idea of being "easy to understand." Therefore, 'clear' is the only option that logically completes the sentence, perfectly aligning with the description of an explanation that is readily comprehensible.