Final answer:
A real gas is expected to deviate from ideal behavior under high pressure with small volume, as well as at low temperature and high pressure. This is because under these conditions, intermolecular forces become significant and the volume of gas particles cannot be ignored. The correct option for when a real gas deviates from ideal behavior is D. I and III both.
Step-by-step explanation:
The question pertains to the conditions under which a real gas is expected to deviate from ideal behavior as dictated by the ideal gas law. According to the kinetic-molecular theory, a gas behaves ideally when the particles occupy negligible volume and no intermolecular forces are acting between them. These conditions are most closely approximated at high temperatures and low pressures.
At high pressure and small volume (condition I), a gas is forced into a smaller space, and intermolecular interactions become significant, causing deviations from ideal behavior. At low temperatures and high pressure (condition III), the kinetic energy of gas particles decreases, resulting in more substantial intermolecular attractions and a deviation from ideal behavior. Conversely, at high temperatures and low pressure (condition II), real gases behave most like ideal gases because the increased kinetic energy tends to overcome intermolecular attractions, and the particles are far enough apart that their volume is less significant. Therefore, the conditions under which a real gas is expected to deviate from ideal behavior are high pressure, small volume, and low temperature, high pressure. The mentioned correct option in the final part of the answer is D. I and III both.