Which instrument's range is C2-C6?

The cello is indeed known for having a range that extends from C2 to C6. As a string instrument, the cello produces lower pitches, beginning with C2, which is its lowest note, and extending up to C6, which allows for a significant range suitable for both melodic and harmonic playing.

In the context of the options provided, each of the other instruments listed has a different range. The tuba, for instance, typically has a range that starts around the lowest C, often not extending as high as C6. The violin, known for its higher pitch range, usually spans from G3 to around E7, while the viola, like the cello, occupies a middle range but generally extends from C3 to around E6.

Therefore, the cello's established range of C2 to C6 makes it the correct choice in this instance.