What are the typical performing forces in a string quartet?

A string quartet is traditionally composed of four string instruments, which are typically two violins, one viola, and one cello. This specific arrangement of instruments allows for a rich tapestry of sound and provides a balance between melody, harmony, and texture. The first violin often carries the main melody, the second violin usually supports with harmonies or counter-melodies, the viola enriches the harmonic structure with its middle register, and the cello anchors the group with its deeper tones, providing both bass support and occasional melodic contributions.

The other combinations of instruments listed do not fit typical string quartet instrumentation. For instance, including a piano results in a different ensemble known as a piano quartet, while woodwind and brass instruments listed in the other options significantly alter the ensemble's character and sound, moving away from the core string-focused arrangement that defines a string quartet. Therefore, the answer accurately represents the traditional setup of a string quartet as understood in classical music.