Final answer:
The root-mean-square (rms) speed of molecules in a gas is that speed such that half the molecules are moving faster than the rms speed and the other half moving slower. It is calculated based on the temperature and molar mass of the gas and is related to the kinetic energy of the molecules.
Step-by-step explanation:
The root-mean-square (rms) speed of molecules in a gas is defined as the square root of the average of the squares of the speeds of the molecules. It is linked to the kinetic energy of the gas molecules and can be calculated from the temperature and the molar mass of the gas. The rms speed is different from the average speed or the most probable speed of the molecules that can be found using the Maxwell-Boltzmann distribution.
Regarding the choices provided in the student's question, option B is correct. That is because by definition of the rms speed, it is such that half of the molecules have a speed less than the rms speed, and the other half have a speed greater than the rms speed. It should not be confused with the most probable speed, which is where the peak of the Maxwell-Boltzmann distribution occurs and is lower than the rms speed.