Boyle's Law is one of constant:

Boyle's Law states that for a given mass of confined gas at a constant temperature, the pressure of the gas is inversely proportional to its volume. This means that as the volume decreases, the pressure increases, provided the temperature remains unchanged. Therefore, the constant in Boyle's Law is temperature.

In the context of the choices provided, when temperature is held constant, the relationship between pressure and volume can be observed and described mathematically. This highlights the importance of temperature as a defining factor in Boyle's Law, as it ensures that any changes in pressure or volume can be attributed to their direct interaction without the influence of thermal variations.