How do i...how to copy one datatable column to another column in same datatable

We have no idea where dt1 and dt2 came from - there is no need to clone your datatable to get at the data, just loop through your existing rows and add new rows to the end of table for each of the existing rows.

You need to capture the current size of the datatable and not use dt.Rows.Count in your for loop - why? Because you are going to add more rows to your datatable.

Your code could look like this

using System;
using System.Data;

public class Program
{
	public static void Main()
	{
		
		Demo d = new Demo();
		//Create our demo table and populate it with marks for A,B,C,D
		DataTable dt = d.createDt("demo");

		//Capture the CURRENT size of the DataTable
		int dtsize = dt.Rows.Count;
		
		for (int k = 0; k < dtsize; k++)
		{
			DataRow dr;
			dr = dt.NewRow();
        	dr["name"] = dt.Rows[k]["mark"];
        	dt.Rows.Add(dr);
		}

		for (int k = 0; k < dt.Rows.Count; k++)
			Console.WriteLine("{0} : {1}\n", k, dt.Rows[k]["name"]);
	}

}

A word of warning about the line

dr["name"] = dt.Rows[k]["mark"];

If you have created your DataTable with the marks column as an integer then you should really convert the mark to a string before inserting it into a name i.e.

dr["name"] = dt.Rows[k]["mark"].ToString();