The correct option is A He is busy today, so he cannot attend to any more clients.
A compound sentence is made up of independent clauses that can stand on
their own as sentences. Illative compound sentences contain clauses where one is inferred or concluded from the other clause. The given sentences are Illative in nature, "He cannot attend... ." is concluded from "He is busy today." "So" is an Illative conjunction that refers to the conclusion mentioned, hence option A is correct as it contains the appropriate illative conjunction. Option B contains the copulative conjunction "and". Option C contains a complex sentence, where "because" is a subordinating conjunction. Hence options B and C are incorrect. Option D is incorrect because the conjunction "or" is alternative and not illative.