Explanation:

- Robust: The word "robust" typically describes something strong, healthy, or vigorous.

- Agile (Option 1): Refers to the ability to move quickly and easily; not directly opposed to robustness in strength or health.

- Healthy (Option 2): Describes a state of good health, similar to robustness.

- Weak (Option 3): The opposite of strong or robust, implying fragility or lack of strength. This is the correct antonym.

- Strong (Option 4): Similar in meaning to robust, both indicating strength and vigor.

Given these explanations, the antonym of "robust" is clearly "weak."

- Correct Answer: Weak (Option 3)