Final answer:
A teachable moment is when a teacher briefly digresses from the planned lesson to explain a concept or event that has captured students' collective interest.
Step-by-step explanation:
A teachable moment is best characterized by option B: The teacher briefly digresses from the planned lesson to explain a concept or event that has captured students' collective interest. In this scenario, the teacher recognizes an opportunity to seize the students' engagement and curiosity and takes a detour from the planned lesson to provide additional information and foster meaningful discussion.