Final answer:
Decreasing both the temperature of water and the pressure of the gas above it will decrease the equilibrium concentration of a non-reacting gas in the water, while decreasing the volume of water is not relevant in this context.
Step-by-step explanation:
Actions that will decrease the equilibrium concentration of a non-reacting gas in water include decreasing the temperature of the water and decreasing the pressure of the gas above the liquid. Decreasing the temperature of water decreases the kinetic energy of gas molecules, which reduces their solubility and thus equilibrium concentration in water. Conversely, decreasing the pressure of the gas above the liquid reduces the number of gas molecules that can be dissolved in water, again leading to a lower equilibrium concentration of the gas in the solution.
Decreasing the volume of water does not affect the equilibrium concentration of a gas dissolved in the water. Rather, changes in volume affect the pressure and can impact the position of equilibrium in reactions involving gases.
In conclusion, the actions that will decrease the equilibrium concentration of a non-reacting gas in water are decreasing the temperature and decreasing the pressure of the gas, corresponding to options (a) 1 only and (d) 1 and 3.