Answer:
Reference
Explanation:
The Reference type variable is such type of variable in C# that holds the reference of memory address instead of value. Examples for reference type are classes, interfaces, delegates and arrays. 
 We can pass parameters to the method by reference using <em>ref </em>keyword 
 It’s mandatory to initialize the variable value before we pass it as an argument to the method in c#
For example, 
int x = 10;  // Variable need to be initialized
Add(ref x); // method call
If you pass parameters by reference in method definition, any changes made to it affect the other variable in method call. 
Here's a sample program:
using System;
namespace ConsoleApplication
{
    public class Test
    {
        public static void Main()
        {
            int i = 10;
            Console.WriteLine("i=" + i);
            Add(ref i);
            Console.WriteLine("i=" + i);
            Console.ReadLine();
        }
        public static void Add( ref int j)
        {
            j = j + 10;
            Console.WriteLine("j="+j);
        }
    }
}
Output: 
i=10
j=20
i=20