Step 1: Concept
The root-mean-square (rms) speed is the measure of the speed of particles in a gas, which is the square root of the average of the squares of the velocities.
Step 2: Meaning
It represents the speed of a molecule possessing average kinetic energy.
Step 3: Analysis
From the kinetic theory of gases, the average kinetic energy of a mole of gas is $\frac{3}{2}RT$. Since kinetic energy is also $\frac{1}{2}Mv^{2}$, equating the two gives $v_{rms} = \sqrt{\frac{3RT}{M}}$.
Step 4: Conclusion
Therefore, the correct mathematical expression for rms speed involves the square root of three times the product of the gas constant and temperature, divided by molar mass.
Final Answer: (A)