//存放数据的集合
            var topicList = db.Topics.ToList();

            //要转存的集合
            List<getTopic> lgt = new List<getTopic>();

            //遍历存放数据集合的数据
            foreach (var item in topicList)
            {
                //要转存的对象（模型类）
                getTopic gt = new getTopic();


                //对象中的属性对应
                gt.TID = item.TopicId;
                gt.TExplain = item.TopicExplain;
                gt.TScore = item.TopicScore;
                gt.TType = item.TopicType;
                gt.TAnswer = item.TopicAnswer;
                gt.TpaperId = item.PaperId;
                gt.TSort = item.TopicSort;

                List<TopicSelect> selects = new List<TopicSelect>();
                selects.Add(new TopicSelect { Tselect = "A", topics = item.TopicA });
                selects.Add(new TopicSelect { Tselect = "B", topics = item.TopicB });
                selects.Add(new TopicSelect { Tselect = "C", topics = item.TopicC });
                selects.Add(new TopicSelect { Tselect = "D", topics = item.TopicD });
                gt.topicSelect = selects;

                //将数据添加进要转存的集合中
                lgt.Add(gt);
            }

---

对应的模型类

public class TopicSelect
    {
        /// <summary>
        /// 选项标识
        /// </summary>
        public string? Tselect { get; set; }
        /// <summary>
        /// 选型内容
        /// </summary>
        public string? topics { get; set; }
    }

    public class getTopic
    {
        /// <summary>
        /// ID
        /// </summary>
        public int TID { get; set; }
        /// <summary>
        /// 描述
        /// </summary>
        public string? TExplain { get; set; }
        /// <summary>
        /// 分数
        /// </summary>
        public int? TScore { get; set; }
        /// <summary>
        /// 类型
        /// </summary>
        public int? TType { get; set; }
        /// <summary>
        /// 排序
        /// </summary>
        public int? TSort { get; set; }
        /// <summary>
        /// 答案
        /// </summary>
        public string? TAnswer { get; set; }
        /// <summary>
        /// 试卷ID
        /// </summary>
        public int? TpaperId { get; set; }
        /// <summary>
        ///选项
        /// </summary>
        public List<TopicSelect>? topicSelect { get; set; }
    }

---

优化一下

            var topicList = db.Topics.ToList();
            List<getTopic> lgt = topicList.Select(item => new getTopic
            {
                TID = item.TopicId,
                TExplain = item.TopicExplain,
                TScore = item.TopicScore,
                TType = item.TopicType,
                TAnswer = item.TopicAnswer,
                TSort = item.TopicSort,
                TpaperId = item.PaperId,
                topicSelect = new List<TopicSelect>
                {
                    new TopicSelect { Tselect = "A", topics = item.TopicA },
                    new TopicSelect { Tselect = "B", topics = item.TopicB },
                    new TopicSelect { Tselect = "C", topics = item.TopicC },
                    new TopicSelect { Tselect = "D", topics = item.TopicD }
                }


            }).ToList();