Concept:The correct relative pronoun must show possession because the dog belongs to the neighbour.
Explanation:The sentence means: My neighbour has left town, and his dog barks every night.
The dog belongs to the neighbour, so we need a possessive form.
The word "which" refers to things and cannot show possession here.
The word "who" is a subject pronoun, not a possessive pronoun.
The word "who's" is a contraction of "who is" or "who has", so it is not possessive.
The word "whose" is the possessive relative pronoun, which fits the blank correctly.
The completed sentence is: "My neighbour, whose dog barks every night, has left town."
Answer:D. whose