The correct option is A agitated
The word 'serene' refers to something calm, peaceful and untroubled.
Option A, nervous refers to being easily agitated or alarmed.
Option B, jocular refers to being humorous or playful.
Option C, earnest results from showing sincere and intense conviction.
Option D, agitated refers to feeling troubled or nervous.
Thus, according to the meaning, the word 'agitated' is the opposite of the word 'serene'.