LQ3

using System;
using System.Collections.Generic;
using System.Linq;
using System.Drawing;
using System.Text;

namespace ClassWork
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            foreach (var line in ThirdTask(new[] {"abc", "bgf", "aab", "Ffg", "fdgOkM"}))
            {
                Console.WriteLine(line);
            }
        }

        public static string[] ThirdTask(string[] lines)
        {
            var letterCounter = lines
                .Select(line => line.ToLower().ToCharArray()
                    .GroupBy(symb => symb)
                    .Select(group => Tuple.Create(group.Key, group.Count())))
                .Where(dict => dict.All(x => x.Item2 == 1));

            return letterCounter
                .Select(tuple =>
                {
                    var str = new StringBuilder();
                    foreach (var x in tuple) str.Append(x.Item1);
                    return str.ToString();
                })
                .ToArray();
        }
    }
}