The correct option is A Who did Doran think was probably the best English actor?
The quotation marks around "perhaps the greatest of English actors," tell us that the author is quoting from the book he is reviewing, and hence the author of that book, Doran, thinks Betterton was probably the best. Doran writes long and interesting records of the triumphs of Garrick but we cannot infer Dorans opinion of the actor from that. The author would not give a definite answer to the question because he says the writings of Doran do not relieve one of the doubt