The correct option is A most beautiful
When a positive is changed to a superlative, and the positive is of three or more syllables, then we use the superlative 'most'., without changing the form of the noun. Hence option A is correct. Option B is incorrect because when nouns of three syllables and more are changed to a superlative, then 'est' is not added to the noun. Option C is incorrect because the phrase is a positive. Option D is incorrect because 'very' is not a superlative.