The correct response is:
the gas constant of each gas
The gas constant (R) is a property that can differ between different gases, reflecting their characteristics. However, it's important to note that under the ideal gas law (PV = nRT), the quantity that ultimately affects the specific behavior or amount of gas present at the same conditions (pressure, volume, and temperature) is the number of moles, which can also differ.
So if your goal is to focus strictly on the properties that vary based on the type of gas in identical conditions, "the gas constant of each gas" would be the appropriate choice. However, if operationally considering the scenario of equal pressure, volume, and temperature directly leading to differences observed between the gases, "the number of moles of each gas" is also a valid choice due to differences in molar mass leading to different quantities of each gas being present in those conditions.
In a strict sense of your question, the molar mass and the number of moles could also differ based on the type of gas. Nevertheless, if you need only one definitive answer, "the gas constant of each gas" can be highlighted as correct.