Which of the following best describes the characteristics of a string quartet?

A string quartet is a specific ensemble that typically consists of four string instruments. The standard configuration for a string quartet is indeed two violins, one viola, and one cello. This combination allows for a rich and harmonious blend of textures and timbres, enabling a variety of musical expressions.

The presence of two violins contributes to the melodic and harmonic layers, while the viola offers a deeper, warmer sound, and the cello provides a strong bass foundation. This arrangement is not only traditional but also serves as the basis for numerous classical compositions, particularly during the Classical and Romantic periods.

Other configurations, such as those listed in the other options, do not adhere to the accepted definition of a string quartet. For example, having a double bass or two violas would alter the overall sound and structure intended in a string quartet, making them incorrect choices. The essence of the string quartet lies in its characteristic balance of the four voices, which is precisely captured in the correct answer.