The correct answer is $\boxed{\frac{{200}}{{\sqrt 3 }}{\text{ V}}}$.
In a three-phase generator, there are three coils that are displaced by 120 degrees from each other. The voltage induced in each coil is the same, but the phase angle of the voltage is different. The voltage between any two terminals of the generator is the vector sum of the voltages in the two coils.
The r.m.s. value of the voltage between any two terminals is $\frac{{200}}{{\sqrt 3 }}{\text{ V}}$.
Option A is incorrect because it is the r.m.s. value of the voltage in each coil. Option B is incorrect because it is the peak value of the voltage in each coil. Option C is incorrect because it is the r.m.s. value of the voltage between two terminals that are 120 degrees apart.