B. That dog gets fatter every day. Check it out:
A is incorrect because of the tense of fuller combined with the word "more." The correct tense would be, "more full." C is incorrect for the same reason. The correct tense would remove the "most" and leave the word "quickest." D also applies for the same reason. We once again would either delete the "most" in that sentence, or we would change the tense of the verb to "loud."