The correct option is B That bird is very beautiful.
When an exclamatory sentence is transformed to an assertive sentence, the structure is changed, without altering its meaning. The structure of the given sentence is : How + adjective (beautiful) + subject (that bird) +verb! (is!), which changes to the assertive form: subject + verb + very + adjective. ('How' is replaced by 'very' and it is used after the verb). Option B is of the required structure and means the same as the original sentence, hence B is the correct answer. Option A is incorrect because the meaning of this sentence is opposite to that of the original, it says the bird is not beautiful, when in fact the original sentence calls the bird beautiful. Option C is incorrect because the sentence is interrogative in form. Option D is incorrect because this sentence is not assertive, it's of the superlative degree of comparison and the sentence is incomplete, the adjective 'most beautiful' is not followed by the noun.