Which instrument produces the sound of a single frequency?

NO musical instrument produces a 'pure' tone with only a
single frequency in it.

EVERY instrument produces more or less harmonics (multiples)
in addition to the basic frequency it's playing.

The percussion instruments (drums etc) are the richest producers
of bunches of different frequencies.

Fuzzy electric guitars are next richest.

The strings and brass instruments are moderate producers of
harmonics ... I can't remember which is greater than the other.

Then come the woodwinds ... clarinet, oboe, etc.

The closest to 'pure' tones of single frequency are the sounds
made by the flute and piccolo, but even these are far from 'pure'.

The only way to get a true single-frequency sound is from an
electronic 'sine wave' generator.