The sentence which uses possessive personal pronouns correctly is B.
These are the correct ways the other sentences should written.
A. Ewan wondered if the keys were his. (there is no need for that apostrophe)
C. The dog scratched its fur over and over. (The way it is written is it's which is a contraction for it is.) It wouldn't sound right if you said the dog scratched it is fur.
D. Theirs are the two on the side, by the counter. (There is no need for that apostrophe)
B. Have you seen the movie and its sequels? (the possessive pronoun its it correct because if it was it's then the sentence would read as follows. Have you seen the movie and it is sequels.)
Hope this helps. :)