Concept:The correct preposition forms the idiom "sit about," which means to remain idle or do nothing.Explanation:The sentence "Get to work!" tells someone to be active.So the missing phrase should mean staying inactive."Sit about" means to sit doing nothing or waste time.Thus the complete sentence is: "Get to work! Don't sit about doing nothing."The options "up," "on," and "by" do not form this natural expression in this context.Answer:A. about