The statement that best describes why the bell is seen vibrating but not heard is:
Light does not need a medium to travel through, but since sound waves must have a medium to vibrate, sound is not created where no air is present.
This statement captures the essence of the phenomenon: sound waves are mechanical waves that require a medium (like air, water, or solid materials) to propagate, whereas light waves are electromagnetic waves that can travel through a vacuum. Hence, even though the bell is vibrating, the absence of air in the vacuum jar means there are no sound waves produced that can travel to our ears.