Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of punctuation between a main clause and a supplementary element. In this choice, the comma is used correctly to mark the boundary between the main clause ("This...Act") and the supplementary relative clause ("which...designs") that provides additional information about the Engravers’ Copyright Act.
Why the other choices are wrong
Choice B
Choice B is incorrect because a semicolon can’t be used in this way to mark the boundary between a main clause and a supplementary relative clause. A semicolon is conventionally used to join two main clauses, and "which...designs" isn’t a main clause.
Choice C
Choice C is incorrect because it results in a rhetorically unacceptable sentence fragment.
Choice D
Choice D is incorrect. Joining the main clause and the additional information about the Engravers’ Copyright Act ("gave...designs") without the comma and relative pronoun "which" results in a confusing and ungrammatical sentence.