Choose the odd one out: Guitar, Violin, Flute, Sitar.

The guitar, the violin and the sitar all sound because stretched strings are set trembling, whether plucked with the fingers or drawn upon with a bow. The flute sounds because a column of air inside the tube is set trembling by the player's breath, and it carries no strings at all. Bowing does set the violin a little apart from the other two, but that split leaves two instruments on one side and so cannot mark out a single odd one.