Answer:
B) I think the song is beautiful
Step-by-step explanation:
A sentence fragment is simply a collection of words that are similar to a sentence but lack the verb or subject that would make them one, they usually do not convey a complete thought, and are dependent.
So, to make the sentence, Think the song is beautiful into a complete sentence, it needs a noun or pronoun to make it express a complete thought, so the addition of the personal pronoun "I" is enough to make it a complete sentence, so option B is correct.