Final answer:
The pressure of a gas is measured in units of pressure, so from the given choices, options b (101 Pa) and c (21 mmHg) are correct as they represent pressure measurements.
Step-by-step explanation:
The pressure of a gas can be described in terms of units that measure pressure, such as pascals (Pa), millimeters of mercury (mmHg), and atmospheres (atm). When looking at the choices provided by the students, options b (101 Pa) and c (21 mmHg) directly specify pressure measurements. The other options provided do not describe pressure; a (350 K) indicates a temperature, d (0.25 mol of CO) denotes an amount of substance, and e ('the volume of the container') refers to volume, all of which are different properties relevant to gases but are not measurements of pressure.