Equipartition Principle of Energy
The equipartition principle of energy states that, at thermal equilibrium, each degree of freedom in a molecule has an average energy of (1/2)kT, where k is Boltzmann's constant and T is the temperature in Kelvin. In other words, the energy is equally distributed among all possible modes of motion.

Molar Heat Capacity at Constant Volume
The molar heat capacity at constant volume, denoted as Cv, is the amount of heat required to raise the temperature of one mole of a substance by one Kelvin while keeping the volume constant. It represents the energy required to increase the internal energy of the system without performing work or changing the volume.

Trend in Molar Heat Capacity
According to the equipartition principle, the molar heat capacity at constant volume depends on the number of degrees of freedom of a molecule. In general, a molecule with more degrees of freedom has a higher molar heat capacity.

CO2(g)
Carbon dioxide (CO2) is a linear molecule consisting of one carbon atom and two oxygen atoms. It has three degrees of freedom: two translational (motion in x, y, and z directions) and one vibrational (stretching of the C-O bonds). Therefore, CO2 has a total of three degrees of freedom and a relatively low molar heat capacity at constant volume.

SO2(g)
Sulfur dioxide (SO2) is a bent molecule with one sulfur atom and two oxygen atoms. It also has three degrees of freedom: two translational and one vibrational. Therefore, SO2 has the same number of degrees of freedom as CO2 and a similar molar heat capacity at constant volume.

H2O(g)
Water (H2O) is a bent molecule with two hydrogen atoms and one oxygen atom. It has three translational degrees of freedom, two rotational degrees of freedom, and two vibrational degrees of freedom. Therefore, H2O has a total of seven degrees of freedom, which is significantly higher than the degrees of freedom of CO2 and SO2. Consequently, H2O has a higher molar heat capacity at constant volume compared to CO2 and SO2.

Conclusion
Based on the equipartition principle, the molar heat capacity at constant volume for CO2(g), SO2(g), and H2O(g) follows the trend:
CO2 = SO2 < />
In other words, the molar heat capacity of CO2 and SO2 is the same, while H2O has a higher molar heat capacity. Therefore, option 'B' is the correct answer.