The correct Exclamatory sentence for the given Assertive one "The peacock dances beautifully" is:

How beautifully the peacock dances!

Here's why:

• "Truly the peacock dances beautifully!" is not grammatically correct.
• "What a beauty! The peacock dances." is not exclamatory and does not directly express an exclamation about the peacock's dance.
• "Oh! The peacock's dance is beautiful." is exclamatory but lacks the directness and focus on the peacock's dancing action.
• "How beautifully the peacock dances!" uses the exclamatory interjection "How" to emphasize the speaker's admiration for the peacock's dance. It is grammatically correct, expresses an exclamation, and directly focuses on the beauty of the dance.