using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var domain = new string[] { "A", "B", "C", "D" };

        var tests = new List<Test>()

        {

            new Test() {Lists = new List<string>() { "A", "B" } },

            new Test() {Lists = new List<string>() { "A", "C" } },

            new Test() {Lists = new List<string>() { "C" } }

        };

        var missing = FindMissingElements(tests, domain);

        missing.ToList().ForEach(Console.WriteLine);

    }

    private static IEnumerable<string> FindMissingElements(IEnumerable<Test> tests, IEnumerable<string> domain)

    {

       var sourceElements = tests.SelectMany(test => test.Lists).Distinct();

       var missing = domain.Except(sourceElements);

       return missing;

    }

}

​

public class Test {

    public List<string> Lists {get; set;}

}