What is the prepositional phrase in this sentence?

Late last night their was aloud knock at the door

a.) Late last night

b.) there was

c.) a loud knock

d.) at the door

The prepositional phrase in the sentence stated above is: d.) at the door. As the name suggests, prepositional phrase includes a preposition which is located at the beginning of the phrase as its determinant and followed by an object it modifies.