The correct answer is A. The balloon was tied to the post before it floated away.
Explanation:
In grammar, a modifier is a word or set of words that describe a noun or any other element. Additionally, a misplaced modifier occurs as the modifier has not been placed beside the word it modifies and therefore the meaning is unclear as the reader does not know which word is the modifier describing. In the case presented, the sentence with a misplaced modifier is "The balloon was tied to the post before it floated away" because in this case the modifier "before it floated away" has not placed beside the word it modifies (the ballon) and therefore it is not clear whether it was the post or the ballon the one that floated away.
