An effective teaching aid should engage the senses and support the lesson objective.
Why B fits: An aid that activates relevant sensory channels can make an abstract idea easier to observe and understand.
Why the others are incomplete: Attractive colours, visibility, and ease of preparation matter, but they do not guarantee that the aid improves learning.
Teaching point: Select an aid for its educational purpose, make it accessible to all learners, and connect it to explanation, discussion, and feedback.