Final answer:
The distance between the coils in a pair of Helmholtz coils is the same as the radius of the coils, which would be option C. R.
Step-by-step explanation:
The distance between the coils in a pair of Helmholtz coils is designed such that it is equal to the radius of either coil. Thus, if the radius of the coils is R, then the distance between the coils would be C. R. This configuration helps to create a nearly uniform magnetic field in the region between the two coils, which is a valuable property for various experimental setups in physics.