The correct option is D The more they earn, the more they spend
Let’s examine the sentence: “The more they earn, more they spend.” The second clause of the sentence is imitating the structure of the first clause with only difference being in the action verbs. Hence, the second clause would also have the article “the” before “more.” So, the improved sentence would be, “The more they earn, the more they spend.” Hence, Option C is the correct answer.