Explanation:

- The word "sober" often means not under the influence of alcohol but can also refer to being serious and calm in demeanor.

- Option 1: Nervous - Typically suggests anxiety or apprehension, which isn't a direct opposite of "sober."

- Option 2: Agitated - Implies restlessness or an excited state. While it contrasts with calmness, it isn't the best antonym for "sober."

- Option 3: Serious - Shares a similar meaning with "sober" in terms of demeanor, making it not an antonym.

- Option 4: Calm - Reflects tranquility. While "sober" can mean serious, "calm" isn't a direct antonym. However, it contrasts better with states of inebriation or emotional intensity.

- Correct Answer: "Agitated" for context relating to demeanor.