gistfile1.txt

using System;
using System.Collections.Generic;

static class P
{
    static void Main()
    {
        List<Question> question = new List<Question>
        {
            new Question {
                Details = new List<Details>
                {
                    new Details { Id = 1, Description = "abc"},
                    new Details { Id = 2, Description = "def"},
                },
                Id = 3, Name = "ghi", SelectedRatingDescriptionId = 4,
            },
            new Question {
                Details = new List<Details>
                {
                    new Details { Id = 5, Description = "jkl"},
                    new Details { Id = 6, Description = "mno"},
                },
                Id = 7, Name = "pqr", SelectedRatingDescriptionId = 8,
            }
        };
        List<Class1> class1 = new List<Class1>
        {
            new Class1 { QuestionId = 3, RatingDescId = 9  },
            new Class1 { QuestionId = 7, RatingDescId = 10 },
        };

        question.ForEach(q =>
        {
            class1.ForEach(c =>
            {
                if (q.Id == c.QuestionId)
                    q.SelectedRatingDescriptionId = c.RatingDescId;
            });
        });

        // now show the orders
        foreach(var q in question)
        {
            Console.WriteLine("Question: " + q.Id);
            foreach(var d in q.Details)
            {
                Console.WriteLine("Detail: " + d.Id);
            }
        }
        foreach (var c in class1)
        {
            Console.WriteLine("Class1 for Q: " + c.QuestionId);
        }
    }
}
public class Question
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Details> Details { get; set; }
    public int SelectedRatingDescriptionId { get; set; }
}

public class Details
{
    public int Id { get; set; }
    public string Description { get; set; }
}

public class Class1
{
    public int QuestionId { get; set; }
    public int RatingDescId { get; set; }
}