Assertion: A bell ringing in an evacuated jar cannot be heard.
Reason: Sound waves in a gas are longitudinal.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is incorrect

Hint: In order to solve this problem, we need to know about the classification of waves, sound, and the characteristics of a sound wave. We must also know the effect of medium on the propagation of sound waves. Then it would be easier for us to address the assertion and its relation to the given reason.

Complete answer:
Waves are disturbances created in a field. They are broadly divided into three types. They are Mechanical waves, Electromagnetic waves, and Matter waves. In this answer, we will limit ourselves to Mechanical waves.
Mechanical waves are formed with the oscillation of matter. They are further divided into two kinds. They are transverse waves and longitudinal waves. Transverse waves are those waves in which the particle displacement is perpendicular to the direction of propagation of the wave. That is the particles of the wave do not move along with the wave, they only oscillate about their mean position. The best example of transverse waves is the ripples formed in the water. Here the water molecules move up and down, which is seen as crest and trough. But the water molecules do not move forward.
Longitudinal waves are waves where the particle displacement is parallel to the direction of propagation of the wave. That is the particle oscillating between two regions called compression and rarefaction. These waves are seen during tremors in an earthquake.
Sound is also a longitudinal wave. In solids, it can travel as both longitudinal and transverse waves. Therefore, being a mechanical wave, which needs a medium for transfer of energy the sound cannot travel in a vacuum or evacuated space.

So, the correct answer is “Option A”.

Note:
The vacuum does not have any particles for the formation of compressions and rarefactions needed for the propagation of sound. Though sound waves can be longitudinal and transverse, both in the case of solid, we mostly consider them to be longitudinal. The major difference between a mechanical and electromagnetic wave is the medium needed for propagation. EM Waves like light don’t need a medium, i.e. why light from the sun reaches earth.