Step 1 - Understanding the relation between solubility and temperature
The solubility of a substance is defined as the amount of that substance that can be dissolved in 100g of the solvent (usually water, but not necessarily).
The solubility varies with temperature for all substances: for some it varies intensely, for others it varies in a more straight fashion. The variation of solubility with temperature also depends on the physical state of the substance.
While solids have their solubility increased when the temperature is raised, the solubility of gases decreases with temperature.
Step 2 - Choosing the right alternative
Since the exercise is asking about the solubility of a gas, the correct answer is that it decreases as the temperature increases. Item b) decreases is the correct one.