Question

if the temperature of a container of gas remains constant, how could the pressure of the gas increase? the number of gas molecules in the container increases. the mass of the gas molecules increases. the size of the container increases. the diffusion of the gas molecules increases.

Explanation:

According to the ideal - gas law $PV = nRT$ (where $P$ is pressure, $V$ is volume, $n$ is the number of moles of gas, $R$ is the ideal - gas constant, and $T$ is temperature). When $T$ is constant, if $n$ (number of gas molecules, since $n=\frac{N}{N_A}$ where $N$ is the number of molecules and $N_A$ is Avogadro's number) increases, $P$ will increase assuming $V$ is constant. Increasing the mass of gas molecules doesn't directly increase pressure in a simple way as described here. Increasing the size of the container (volume) would decrease pressure. Diffusion of gas molecules is not directly related to increasing pressure under these conditions.

Answer:

The number of gas molecules in the container increases.