Out of the given options, the stringed instrument is the piano. A piano is a musical instrument that is played by striking its keys with the fingers, which in turn causes strings inside the piano to vibrate and produce sound. The sound is amplified and projected by the piano's wooden soundboard and metal strings. The other options are not stringed instruments: the flute is a woodwind instrument, the trumpet is a brass instrument, the drum is a percussion instrument, and the organ is a keyboard instrument that uses air to produce sound.