The sentence compares the efficiency of learning under different conditions. "Fun" suggests that when learning is enjoyable, it is more efficient, while "drudgery" refers to tasks that are monotonous and tedious, leading to inefficiency in learning.
Option (A), fast ... slow, doesn't make sense in this context because the efficiency of learning is not simply about speed; it’s about engagement and enjoyment, which "fun" and "drudgery" better capture.
Option (B), rapid ... turtle-slow, introduces speed, but the focus should be on the contrast between enjoyable and tedious learning experiences.
Option (C), tedious ... like a joy ride, is incorrect because "tedious" correctly contrasts with "fun," but "like a joy ride" doesn’t fit the intended comparison between effective and ineffective learning experiences.
Option (D), fun ... drudgery, is the best choice as it contrasts the enjoyable aspect of learning with the dull and monotonous aspects.
Thus, option (D) is the correct answer.