Concept:The phrase "it is high time" is followed by a past tense verb to show that an action should happen now or soon.Explanation:The structure is "it is high time + subject + past tense."This past verb form is called the past subjunctive, and it expresses a present or future meaning.It also suggests that the action is already slightly late.The subject here is "we," so the correct past tense verb is "started."Therefore, the completed sentence is: "My mother remarked that it was high time we started preparing supper."Answer:D. started