The solubility of a gas in a liquid increases with:

Decrease in temperature

When it comes to the solubility of gases in liquids, the general trend is that solubility decreases with an increase in temperature. This is because the solubility of most gases in liquids is exothermic, meaning that heat is released when the gas dissolves in the liquid. As a result, when the temperature is increased, the system tries to minimize the excess heat by decreasing the solubility.]\The decrease in solubility with increasing temperature can be explained by the Gibbs free energy equation: \( \Delta G = \Delta H - T\Delta S \). The term \( -T\Delta S \) represents the entropic contribution to the free energy change. As the temperature (\( T \)) increases, the value of \( -T\Delta S \) becomes more significant, leading to a decrease in the solubility (\( \Delta G \) becomes positive).

Therefore, the correct answer is \( \text{(3)} \) Decrease in temperature. The solubility of gases in liquids generally increases with a decrease in temperature.