Explanation:

- To compare one thing to all others using "than," we generally use the comparative form of the adjective.

- Option 1: More nourishing - Correct choice when comparing milk specifically to another or multiple items. Milk is more nourishing than any other food.

- Option 2: Nourishing - Describes milk without comparing to another. Incorrect as a direct substitute for the sentence.

- Option 3: Most nourishing - Generally used when stating milk is at the top of nutrition among a specific group but needs context.

- Option 4: As nourishing - Used for making an equality comparison. Not fitting with the use of "than."