using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var list1 = new List<string> { "Only", "three", "elements"};

        var list2 = new List<string> {"Really", "short"};

        var list3 = new List<string> {"This", "is", "the", "longest", "list"};

        var list4 = new List<string> {"This", "is", "also", "5", "long"};

        var combinedLists = new List<List<string>> {list1, list2, list3, list4};

    var groupsOfListsWithMaxLength = combinedLists

         .GroupBy( l => l.Count)

         .OrderByDescending(g => g.Key)

         .First();

        //null handle if needed

        foreach (var strList in groupsOfListsWithMaxLength )

        {

            string output = String.Join(" ", strList);

            Console.WriteLine(output);

        }

    }

}