Answer:
My friend John, who owns the chocolate Lab puppy, is getting ready to leave for a trip.
Explanation:
"Nonrestrictive modifiers" are <em>words or phrases that follows a noun.</em> It is separated by commas since it presents an information that is not essential to the meaning of the sentence as a whole.
Among the choices above, the choice <em>"My friend John, who owns the chocolate Lab puppy, is getting ready to leave for a trip" </em>uses the commas correctly in order to separate its non restrictive modifier.
The non restrictive modifier in the sentence is "who owns the chocolate Lab puppy." This modifies "My friend John." However, it is not an essential information in order to determine the meaning of the whole sentence since the sentence already mentioned the named John.
Out of all the pronouns, "whom" makes the most sense grammatically. This allows us to narrow it down to A or D.
In order for it to be the object of a preposition, there would have to be a preposition in front of it. "For," however is <em>not</em> a preposition. In fact, there isn't a preposition in the whole question.
This leaves us with option D, which is the correct answer.
You can better tell it's a direct object if you flip the sentence around a bit.
Your sister is waiting <em>for whom</em>?
"Whom" is essentially receiving the action. "Whom" is what your sister is waiting for.
Answer: D. whom; direct object.