In ancient Greece, an Epicurean was a follower of Epicurus, a philosopher whose beliefs revolved around

Choice A is the best answer. The convention being tested is punctuation use between a main clause and a participial phrase. This choice correctly uses a comma to mark the boundary between the main clause (“Epicurus…‘soul’”) and the participial phrase (“positing…absence”) that provides additional information about how Epicurus defined pleasure.

Choice B is incorrect because a colon can’t be used in this way to join a main clause and a participial phrase. Choice C is incorrect because a semicolon can’t be used in this way to join a main clause and a participial phrase. Choice D is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “positing.”