The most appropriate option to change the assertive sentence "I wish I had the wings of a bird" to an exclamatory sentence is (2) "If only I had the wings of a bird!" An exclamatory sentence expresses a strong emotion or excitement, and this option conveys the sense of wishing for something with enthusiasm and emphasis, making it an appropriate transformation.

Option (1), "If only I had been a bird," is more of a conditional statement, expressing a hypothetical situation. It doesn't convey the same sense of desire or longing as the original sentence, making it less suitable for transforming the sentence into an exclamatory one.

Option (3), "If I have wings!" is more of an interrogative statement, asking a question, and it doesn't convey the same emotion as the original sentence.

Option (4), "What wings I had!" is also exclamatory and conveys a sense of surprise or realization, but it changes the meaning of the sentence slightly, suggesting that the speaker already has wings, which is not in line with the original sentence's meaning.