Which of the following musical instruments is a wind instrument?
A. Flute
B. Drums
C. Piano
D. All of these

Hint: Recall the working principle of each instrument. There are instruments which are hybrid, which means they are made to work on the principle of wind but can also work on other principles. Don’t confuse between wind and vibrations. They may sound similar in working but are different.

Complete answer:
Let us see the working of all the three musical instruments and then decide which of them are wind instruments.
Flute: Flute being a musical instrument, produces sound when a stream of air is directed across the holes in the instrument which in response creates a vibration of air at the hole. The airstream creates what is known as a Bernoulli or siphon. This excites the air contained in the usually cylindrical resonant cavity within the flute and thus we hear the music.So, option A is correct, flute is a wind instrument.
Drums: Drum is considered to be a member of the percussion group of musical instruments. In the Hornbostel-Sachs classification system, it is known as membranophone. So, drums are a beat instrument.So, option B is incorrect.
Piano: A piano sounds quite unlike any other instrument. The piano is two different kinds of instrument in one: it's a string instrument, because the sounds are made with strings, but it's also a percussion instrument (like a drum) because the strings make sound when something hits them. Generally, piano is considered a string instrument.
So, option C is also incorrect.

Therefore, only option A is correct.

Note:Remember that though sound is produced due compression and rarefaction of air molecules, all instruments might be considered as wind instruments. Based on their working principle, different instruments are classified as wind instruments, beat instruments and string instruments.