In which situation would gas expand?

Gas expansion occurs primarily when there is an increase in temperature. According to the principles of thermodynamics, when the temperature of a gas rises, its kinetic energy increases. This increased kinetic energy causes the gas molecules to move more vigorously, which pushes them further apart. As a result, the volume of the gas expands.

In the scenario given, when temperature increases, the gas absorbs heat energy, leading to more energetic molecular motion and an increase in the volume occupied by the gas, hence resulting in expansion. This behavior is also described by Charles's Law, which states that the volume of a gas is directly proportional to its temperature when pressure is held constant.

In contrast, an increase in pressure tends to compress gas, while a decrease in temperature slows down the molecular motion and can lead to contraction. Decreasing concentration does not inherently lead to gas expansion, as concentration is a measure of how densely packed the molecules are, which can influence behavior but does not dictate expansion on its own. Thus, the correct answer reflects the fundamental principles of gas behavior in relation to temperature.