You are right, the prepositional phrase in the sentence <em>Late last night there was a loud knock at the door </em>is D) at the door.
The only preposition in the sentence is <em>at, </em>therefore this has to be correct.
In addition. The question itself leads you to the answer: "..is about to add....". This indicates to broaden the explanation of something or some concept, in which it is necessary to clarify a point. Sometimes, instead of writing a similar idea it is a good practice to add a metaphor. These figures of speech go through a path that is good for just about anyone.
Insurance Law is, as the name implies, the body of law pertaining to insurance. This includes insurance policies, insurance claims, insurance regulations and rates, and recently enacted laws, like the Affordable Care Act.
I hope this helps!
