Q. There are two cars parked outside: . . . . . . . . blue one and . . . . . . . . grey one. . . . . . . . . blue one belongs to my neighbour. I don't know who is . . . . . . . . owner of . . . . . . . . grey one.

Explanation by: Mr. Dubey

The blue car and the grey car are mentioned for the first time in the first sentence and they're known only to the speaker at this point. They're indefinite things starting with a consonant sound, hence the article 'a' is used and 'the' isn't. In the second sentence, these nouns are mentioned for the second time, which is why 'the' is used and not 'a'. The noun 'owner' has not been mentioned before but is still specific, as there is only one owner of the grey car, hence the article 'the' is correct, and 'a' isn't. This makes the option B correct, having all the correct articles mentioned.