Answer:

To keep the volume constant, the gas must undergo changes in temperature and pressure that maintain the product of pressure and volume (PV) at a constant value. This is known as Boyle's Law.

Boyle's Law: For a given amount of gas at a constant temperature, the pressure and volume are inversely proportional.

Explanation of the options:

A) 273K and 2 atm:
- This option changes the temperature to 273K (which is 0°C) and the pressure to 2 atm.
- Since the temperature and pressure have changed, the volume will not remain constant.
- Therefore, option A is not the correct answer.

B) 273°C and 2 atm:
- This option changes the temperature to 273°C and the pressure to 2 atm.
- However, the temperature is given in degrees Celsius, which needs to be converted to Kelvin.
- To convert Celsius to Kelvin, we add 273 to the given temperature.
- So, 273°C + 273 = 546K
- Since the temperature and pressure have changed, the volume will not remain constant.
- Therefore, option B is not the correct answer.

C) 546°C and 0.5 atm:
- This option changes the temperature to 546°C and the pressure to 0.5 atm.
- However, the temperature is given in degrees Celsius, which needs to be converted to Kelvin.
- To convert Celsius to Kelvin, we add 273 to the given temperature.
- So, 546°C + 273 = 819K
- Since the temperature and pressure have changed, the volume will not remain constant.
- Therefore, option C is not the correct answer.

D) 0°C and 0 atm:
- This option changes the temperature to 0°C and the pressure to 0 atm.
- However, a pressure of 0 atm indicates that there is no gas present.
- Therefore, there will be no volume and the volume cannot remain constant.
- Therefore, option D is not the correct answer.

Conclusion:
None of the given options keep the volume constant.