The correct option is A His bite is bad, but his bark is worse.
A Complex sentence consists of one Main clause and one or more subordinate clause.
Compound sentences are made up of two or more Principal or Main Clauses.
They are generally formed using coordinating conjunctions like and, or, for, so, but, yet etc.
Option A is correct: As the main and the subordinate clause contrast with each other, hence, use of "but" is accurate here.
Option B and C are incorrect: "And" as well as "or" cannot show the contrast between two independent clauses. Hence, use of "or" is incorrect here.
Option D is incorrect as option A is the correct answer.