Which of these instruments has four strings?

The viola is a string instrument that is bowed, plucked, or played with varying techniques. Slightly larger than a violin, it has a lower and deeper sound. Since the 18th century, it has been the middle or alto voice of the violin family, between the violin (which is tuned a perfect fifth higher) and the cello (which is tuned an octave lower). The strings from low to high are typically tuned to C3, G3, D4, and A4.

The viola was popular in the heyday of five-part harmony, up until the eighteenth century, taking three lines of the harmony and occasionally playing the melody line. Music for the viola differs from most other instruments in that it primarily uses the alto clef. When viola music has substantial sections in a higher register, it switches to the treble clef to make it easier to read.

The viola often plays the "inner voices" in string quartets and symphonic writing, and it is more likely than the first violin to play accompaniment parts. The viola occasionally plays a major, soloistic role in orchestral or chamber music.