For what it's worth, I don't think B's reply is sufficient to create an estoppel if you hold that he has complied with the terms of a contract created by offer, acceptance and consideration. Nor do I think a contractual offer is not made by the advertisement; we are in Carlill v The Carbolic Smoke Ball territory. You will have to work out for yourself what the situation would be if the reward was advertised but the finder never saw the advertisement at all and returned the dog in total ignorance of it and whether the finder's seeing it makes any difference. I am not going to help you on that.
The point of these test questions in law is to get you to explain all the the elements of law exposed by the facts given and the questions of law which arise. It does not really matter if your decision, yes or no, is contrary to some obscure case on exactly the same facts, because it's 100 to 1 that there is no such reported case; otherwise you should get full marks by simply saying "Yes, see X v Y [1975] 2 All England Reports", or whatever the report is