Explanation:

- The word "decisive" implies a clear, firm, and determined action or decision.

- Option 1: decisive - This is the correct option. It is indeed the antonym of doubtful, as decisive means having a clear, firm decision, whereas doubtful implies uncertainty.

- Option 2: played - This word means engaged in activity or performed a function. It does not relate to being doubtful or its antonym.

- Option 3: adult - This refers to someone who is fully grown or developed. It is unrelated to doubtfulness.

- Option 4: poverty - It refers to the state of being extremely poor; not opposite of doubtful.