Choose the best word: His explanation was so _____.

His explanation was so lucid. This choice indicates that his explanation was clear and easy to understand, which is typically the goal of effective communication. When someone provides a lucid explanation, they break down complex ideas into simpler terms, making it accessible to the audience. This is particularly important in competitive exams, where clarity of thought and expression can significantly impact the understanding of questions and concepts.

In contrast, the other options present negative connotations regarding clarity. The word "obscure" suggests that the explanation was difficult to understand, while "confusing" directly indicates a lack of clarity that could leave the audience bewildered. "Ambiguous" implies that the explanation could be interpreted in multiple ways, which further detracts from effective communication. These alternatives highlight the importance of precision in language, especially in contexts such as academic settings or competitive exams where clarity can influence performance.

Choosing the best word in this scenario underscores the value of clarity in explanations. A lucid explanation not only demonstrates the speaker's understanding but also facilitates better learning for the audience. In competitive exams, being able to discern the most appropriate words to convey ideas effectively is crucial for success. Thus, selecting "lucid" aligns perfectly with the context provided, emphasizing the significance of clear communication in both academic and everyday settings.