Explanation:

- Marginal Propensity to Save (MPS): This refers to the proportion of any additional income that is saved rather than spent. A higher MPS decreases the Keynesian Multiplier since less income is spent in the economy.

- Marginal Propensity to Consume (MPC): This is the fraction of additional income that is spent on consumption. A higher MPC increases the Keynesian Multiplier as more income circulates within the economy.

- Both (a) and (b): Both MPS and MPC directly affect the Keynesian Multiplier. The multiplier is calculated as 1/(1-MPC) or 1/MPS. Thus, both factors are crucial.

- None of the above: This is incorrect, as both MPS and MPC are integral to determining the multiplier.

Your correct answer is: Option 3: Both (a) and (b)