What phenomenon is observed when air is blown into the cap of a pen?
(A) Light is produced
(B) Sound is produced
(C) Seat is produced
(D) Chemical energy is produced

Hint
When air is blown into a cap of a pen, resonance occurs in the cavity and produces sound. This is known as Helmholtz resonance which is a basic concept behind most of the musical instruments such as flute, guitar, and trumpets.

Complete step by step answer
Sound is produced through the vibration of particles. Vibration is defined as the periodic back and forth motion of particles when force is applied. While moving back and forth the particles bump into each other and produce sound. These particles result in a formation of mechanical waves known as sound waves. Sound is measured in frequency.
When air is blown into the pen cap with pressure a resonance is created in the cavity which produces the sound. The cavity resonance is known as Helmholtz resonance.
Helmholtz resonance is the phenomenon of air resonance in a cavity. The resonance occurs when the stimulus frequency and the natural frequency matches. When resonance occurs vibration is produced.
Similarly, when air is blown into the cap the frequency of the blown air resonates (matches) with the frequency of volume of air in the cap and it produces vibrations, these vibrations cause movement of the surrounding air molecules and make them vibrate and produce sound.
Hence, the answer is Option (B) Sound is produced.

Note
Sound travels through a medium, it cannot travel in vacuum. So the vacuum sound cannot be heard. An interesting fact is that even if the sun explodes or if a galaxy is destroyed in space we cannot hear it. This explains the importance of air in auditory perception.