The activity of an ideal gas is numerically __________ its pressure.

Answer: Option C

Activity $$\left(a\right)$$ of a pure ideal gas is defined as:

$$a=\frac{\text{fugacity at an temperature and pressure of interest}}{\text{fugacity at same temperature and standard pressure }}$$

usually the, standard pressure $$=$$ 1 atm and for ideal gas fugacity at a temperature and pressure $$=$$ pressure at that conditions
So, $$a=P$$