Explanation:

- Option 1: Ratio of saving to income

- This refers to the portion of income that is saved rather than spent. It measures how much of a person’s or household’s income is not used for immediate consumption.

- Option 2: Level of income at which saving is equal to income

- This implies that all income is being saved, which is not typically how "propensity to save" is defined.

- Option 3: Additional income that is not to be saved

- This identifies income that is spent rather than saved but doesn’t directly relate to the "propensity to save".

- Option 4: None of these

- This suggests that none of the above options correctly define propensity to save.

Option 1 is the correct choice.