The correct option is B This
In this case, This, These and Those are the demonstrative adjectives because they are being used with the noun gun. It is a personal pronoun. Pronouns are used instead of a noun. With the existence of noun in the sentence, use of pronoun -it- is eliminated. Between, 'this' 'these' and 'those', 'these' and 'those' are ruled out because they are plural and the gun is singular. This is the correct demonstrative adjective that serves the purpose and indicates that the gun needs oiling.