How are musical instruments grouped?

A: by their frequency

B: by the wavelengths they produce

C: by what they are made of

D: by how they produce pitch

Musical instruments are grouped by how they produce the pitch.

Pitch is the highness or lowness of a tone.

A and B are equivalent. That's one way instruments are often grouped. (the "sopranos", the "altos", the "bass")

C is another way instruments are often grouped; (the "woods", the "brass")

D is another way instruments are often grouped; (the "strings", the "percussions")