When an ideal gas undergoes an isothermal change, its temperature remains constant. According to Boyle`s Law, the pressure and volume of a gas are inversely proportional to each other at constant temperature.

In this question, the pressure of the gas is increased by 10%. Since the temperature is constant, the volume must decrease to maintain the constant pressure-volume product.

To find out the approximate decrease in volume, we can use the concept of proportionality. When the pressure increases by 10%, the volume must decrease by a proportional amount.

Option 1 suggests a decrease of 0.1%. This value is too small compared to the increase in pressure, so it can be eliminated.

Option 2 suggests a decrease of 9%. Since the increase in pressure is 10%, a decrease of 9% is a reasonable estimate for the volume change. This option is the correct answer.

Option 3 suggests a decrease of 10%. Although this seems logical, it assumes that the relationship between pressure and volume is perfectly proportional, which is not always the case for all gases. Therefore, it can be eliminated.

Option 4 suggests a decrease of 0.9%. This value is too small compared to the increase in pressure, so it can be eliminated.