Step-by-step explanation:
D. Paul told us about his resignation: "This is difficult, but I believe it would be best for the students if I step down from my position."
Explanation: A colon is used to introduce a quote or explanation, and in this sentence, the colon is used correctly to introduce Paul's quote about his resignation. Option A is incorrect because colons should not be used after author names. Option B is incorrect because it uses colons excessively and incorrectly. Option C is incorrect because the colon should not be used before a list that is not an independent clause.