LINQ-7

using System;
using System.Collections.Generic;
using System.Linq;

namespace Homework7
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Soldier> troop1 = new List<Soldier>
            {
                new Soldier("К.."),
                new Soldier(),
                new Soldier("А.."),
                new Soldier(),
            };

            List<Soldier> troop2 = new List<Soldier>
            {
                new Soldier("В.."),
                new Soldier(),
                new Soldier("Н.."),
            };

            List<Soldier> selectedSoldiers = troop1.Where(soldier => soldier.Surname.StartsWith("Б")).ToList();

            troop2.AddRange(selectedSoldiers);
            troop2 = troop2.OrderBy(soldier => soldier.Surname).ToList();

            troop1 = troop1.Except(selectedSoldiers).OrderBy(soldier => soldier.Surname).ToList();

            foreach (var soldier in troop2)
            {
                soldier.ShowInfo();
            }
        }
    }

    class Soldier
    {
        public string Surname { get; private set; }

        public Soldier(string surname = "Б..")
        {
            Surname = surname;
        }

        public void ShowInfo()
        {
            Console.WriteLine(Surname);
        }
    }
}