The correct answer is (A).
Getting to the Answer: The question asks which alternative would be unacceptable, so there is no error in the original sentence. The second part of the sentence is parenthetical information that provides additional description about the platform. The correct answer choice will provide an unacceptable way of expressing this information.
(A) Correct: a colon is not used to connect an independent clause to a parenthetical phrase
(B) Incorrect: this choice correctly uses a comma and a FANBOYS conjunction to connect two independent clauses
(C) Incorrect: this choice correctly creates two complete sentences ending with periods
(D) Incorrect: this choice correctly creates two independent clauses connected by a semicolon