C# Online Compiler | .NET Fiddle

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

namespace ConsoleApplication1
{
    public class Employee
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public List<Department> Departments { get; set; }
    }

    public class Department
    {
        public string Name { get; set; }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            List<Employee> employees = new List<Employee>();
            employees.Add(new Employee
            {
                ID = 1,
                Name = "Kapil",
                Departments = new List<Department>()
                {
                    new Department { Name = "Marketing" },
                    new Department { Name = "Sales"}
                }
            });
            employees.Add(new Employee
            {
                ID = 2,
                Name = "Raj",
                Departments = new List<Department>()
                {
                    new Department { Name = "Advertisement" },
                    new Department { Name = "Production"}
                }
            });
            employees.Add(new Employee
            {
                ID = 3,
                Name = "Ramesh",
                Departments = new List<Department>()
                {
                    new Department { Name = "Production" },
                    new Department { Name = "Sales"}
                }
            });

            var result = employees.SelectMany((w, index) => w.Departments.Select(k => index.ToString() + "," + k.Name));
            foreach (var dept in result)
            {
                Console.WriteLine(dept);
            }
        }
    }
}

 
using System;
using System.Collections.Generic;
using System.Linq;
​
namespace ConsoleApplication1
{
    public class Employee
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public List<Department> Departments { get; set; }
    }
​
    public class Department
    {
        public string Name { get; set; }
    }
​
    public class Program
    {
        public static void Main(string[] args)
        {
            List<Employee> employees = new List<Employee>();
            employees.Add(new Employee