C# For Dummies, Chapter 7, Solution 14

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

namespace Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int m = int.Parse(Console.ReadLine());

            var matrix = new string[n, m];
            //Input
            for (int i = 0; i < n; i++)
            {
                for (int j = 0; j < m; j++)
                {
                    matrix[i, j] = Console.ReadLine();
                }
            }

            //Rows
            int tempCount = 1, count = 1;
            string element = string.Empty;
            for (int row = 0; row < n; row++)
            {
                for (int col = 0; col < m - 1; col++)
                {
                    if (matrix[row, col] == matrix[row, col + 1])
                    {
                        tempCount++;
                        if (count < tempCount)
                        {
                            count = tempCount;
                            element = matrix[row, col];
                        }
                    }
                }
                tempCount = 1;
            }

            //Collums
            for (int col = 0; col < m; col++)
            {
                for (int row = 0; row < n - 1; row++)
                {
                    if (matrix[row, col] == matrix[row + 1, col])
                    {
                        tempCount++;
                        if (count < tempCount)
                        {
                            count = tempCount;
                            element = matrix[row, col];
                        }
                    }
                }
                tempCount = 1;
            }

            //Diagonal
            for (int i = 0; i < n - 1; i++)
            {
                if (matrix[i, i] == matrix[i + 1, i + 1])
                {
                    tempCount++;
                    if (count < tempCount)
                    {
                        count = tempCount;
                        element = matrix[i, i];
                    }
                }
            }

            //Output
            while (count > 0)
            {
                Console.Write($"{element} ");
                count--;
            }
        }
    }
}