When a tuning fork vibrates over an open pipe and the air in the pipe starts to vibrate, the vibrations in the tube are caused by
a. harmonics.
b. beats.
c. reinforcement.
d. resonance.

When a tuning fork vibrates over an open pipe and the air in the pipe starts to vibrate, the vibrations in the tube are caused by:

d. resonance.

When the tuning fork vibrates near the open end of the cylinder, the sound waves from the fork are sent into the pipe. When one object vibrates, it forces another object to vibrate at the same frequency and this is called resonance .

Explanation:

In physics, resonance is an event in which a vibrating system or external force drives different system to vibrate with greater amplitude at particular frequencies. Frequencies at which the response amplitude is a relative peak are known as the system's resonant frequencies or resonance frequencies.