ANS:D - depends on the choice of the standard state.

At a given temperature and pressure, the activity of a gas is the ratio of its fugacity to the fugacity of the same gas at the standard state. The standard state is typically defined as a pressure of 1 atm and a specified temperature. For pure hydrogen gas at 1000°C and 5 atm pressure, its activity can be determined using the relationship between fugacity and pressure, which depends on the behavior of the gas at the given conditions. Without specific information about the behavior of hydrogen at this temperature and pressure, it's challenging to determine the exact value of its activity. However, at high temperatures and pressures, gases can behave significantly differently from ideal behavior, leading to deviations from unity in their activities. Therefore, the activity of pure hydrogen gas at 1000°C and 5 atm pressure may not necessarily be equal to 1 and can vary depending on factors such as temperature, pressure, and the specific behavior of hydrogen under those conditions. In summary, the activity of pure hydrogen gas at 1000°C and 5 atm pressure cannot be determined without additional information about its behavior under those specific conditions. Therefore, none of the given options can be definitively chosen without further details.