Explanation:

- The multiplier effect is an economic concept used to determine the impact of an increase in investment or spending on overall economic output.

- Option 1: K = ?S/?I

- This formula suggests a relationship between changes in savings (S) and investment (I), but it's not the standard definition of the multiplier.

- Option 2: K = ?Y/?I

- This formula represents the multiplier correctly. It describes how output (Y) changes with respect to changes in investment (I).

- It's the widely accepted definition for the investment multiplier in economics.

- Option 3: K = I – S

- This expression shows the difference between investment and savings. It's not related to the multiplier.

- Option 4: None of these

- This would be correct if no expression matched the multiplier, but option 2 does.