Sentence A is correct because when you have a conjunctive adverb linking two independent clauses, you should use a semicolon. In this case, both of these independent clauses have been connected by the conjunctive adverb <em>however</em> (see 1 and 2). What is more, you should use a comma after the conjuctive adverb because it appears in initial position in the second clause.
1) She tried to pain the building; (independent clause I)
Mr. Alex heard a strange voice call him, quickly he turned his head to the left side. Suddenly he felt something tap him on his right shoulder but before he was able to turn his head to see it, it was gone...... Tun tun tuuuuunnn