What does Charles' law consider as a constant?

Charles' Law specifically addresses the relationship between the volume of a gas and its temperature, holding the pressure constant. This law states that if a gas is heated, its volume increases, provided the pressure does not change.

In the context of the question, since the pressure is considered a constant in Charles' Law, it emphasizes that any changes in temperature will result in proportional changes in volume, while pressure remains steady. This is foundational in understanding gas behaviors in thermodynamics and is critical in applications involving ideal gases.

The other answers either do not fit the definition of the law or pertain to different gas laws. For example, considering volume as constant would relate to Boyle's Law, and although temperature changes in relation to volume in Charles' Law, it is not regarded as constant. Thus, the right focus in Charles' Law is on maintaining a constant pressure.