The letter C is an appropriate letter for the velocity of light because it is considered a constant.

"'C' a symbol for the velocity (or "speed") of light. One of the fundamental principles of physics is that light always travels at the same velocity in a vacuum, exactly 299 792 458 meters per second or about 670 617 300 miles per hour. Another fundamental principle is that no object can travel faster than light. At velocities that are large fractions of the speed of light, the theory of relativity predicts a variety of strange physical effects. In calculations involving relativity, speeds are customarily expressed as fractions of the speed of light, such as 0.95 c."

From Russ Rowlett "How Many? A Dictionary of Units of Measurement" http://www.unc.edu/~rowlett/units/dictC.html