The correct answer is A. absolute temperature.
The speed of sound in a gas is proportional to the square root of its absolute temperature. This is because the speed of sound is determined by the average kinetic energy of the molecules in the gas, and the average kinetic energy is proportional to the absolute temperature.
The other options are incorrect because they do not affect the speed of sound in a gas. The temperature of a gas does affect the speed of sound, but only indirectly, through its effect on the average kinetic energy of the molecules. The density of a gas does not affect the speed of sound, because the speed of sound is determined by the average speed of the molecules, not by the number of molecules per unit volume. The modulus of elasticity of a gas does not affect the speed of sound, because the speed of sound is determined by the properties of the molecules themselves, not by the properties of the gas as a whole.