This answer is correct because the phrase "Hiking through the forest" is a dangling modifier. A modifier is considered dangling when it does not properly describe or modify the subject of the sentence. In this case, it is unclear who or what is actually hiking through the forest. The sentence suggests that the birds are the ones hiking through the forest, which does not make logical sense. A possible correction could be: "While hiking through the forest, I could hear the birds' songs filling the air with tranquility."
