Option 1 -> S/Y represents the average propensity to save, not the marginal propensity to save.
Option 2 -> C/Y represents the average propensity to consume, not related to marginal propensity to save.
Option 3 -> ΔS is simply the change in savings, not a propensity ratio.
Option 4 -> 1-c correctly represents the marginal propensity to save, where c is the marginal propensity to consume.
Hence, 1-c -> The marginal propensity to save (MPS) is the proportion of additional income that is saved rather than consumed. Since income can only be either consumed or saved (Y = C + S), the marginal propensities must sum to 1. Therefore, MPS = 1 - MPC = 1 - c, where c represents the marginal propensity to consume. For example, if MPC = 0.8 (80% of additional income is consumed), then MPS = 0.2 (20% is saved) -> correct