The molar heat capacity of oxygen gas at STP is nearly 2.5R. As the temperature is increased, it gradually increases and approaches 3.5R. The most appropriate reason for this behaviour is that at high temperatures.

We know that the molar heat capacity of any gas depends on the degrees of freedom of the gas. On increasing the temperature of the gas, we allow more molecules to vibrate about their equilibrium positions, which in turn increases the degrees of freedom of the gas. For this reason, the molar heat capacity increases as the temperature of the gas are increased.

Hence, the correct answer is an option (d).