A phrase means that there are no active verbs involved. So C and D cannot be correct answers. B is also incorrect, as that is the subject, a noun phrase. So the only possible solution is A, 'along the window ledge'. You answer it by asking the question - Where is it crawling? - Along the window edge. And since you are looking for the location, or a place, it has to have an adverb meaning, and since it doesn't have a verb, it is a phrase. A is the correct answer.
