Explanation:

- Robust generally means strong, healthy, or resilient. It implies durability and sturdiness.

- Option 1: Sound - This typically refers to something being in good condition, healthy, or reliable. It shares a similar sense of strength as "robust."

- Option 2: Feeble - This means weak, lacking strength or vigor. It contrasts sharply with "robust," making it an antonym.

- Option 3: Inadequate - This implies insufficiency or not being enough, but it doesn’t directly oppose the idea of strength or health.

- Option 4: Militant - This conveys aggression or a combative nature. It's not related to strength in the sense that "robust" is, so it’s not an antonym.

Option 2: Feeble is the nearest antonym to "robust."