Explanation:

- The multiplier effect is an economic concept that describes how an initial change in spending can lead to a larger change in overall economic output.

- Option 1: ?Y/?S - This is the change in output (Y) over the change in saving (S). This is not typically used to describe the multiplier effect.

- Option 2: ?Y/?I - This represents the change in output (Y) over the change in investment (I). This is the correct way to describe the investment multiplier.

- Option 3: ?I/?Y - This is the change in investment (I) over the change in output (Y). It does not describe the multiplier effect.

- Option 4: ?Y/?C - This indicates the change in output (Y) over the change in consumption (C). This is not the multiplier effect’s typical representation.