Alright — let's do this. This question relates to Boyle's Law, which describes the relationship between the pressure and volume of a gas at a constant temperature. Boyle's Law states that for a fixed amount of gas at a constant temperature, the pressure ($P$) and volume ($V$) are inversely proportional. This means that if one quantity increases, the other must decrease to keep their product constant. Mathematically, this is expressed as: $$P_1V_1 = P_2V_2$$ or $$P \propto \frac{1}{V}$$ If the pressure of the gas increases while the temperature remains constant, the volume must decrease to maintain this inverse relationship. Therefore, to experience an increase in pressure of a gas at a constant temperature, the volume must decrease. The correct option is (D). The final answer is $\boxed{\text{D) decrease}}$. Got more? Send 'em.