Input
====

A list of lists of letters like:

 ["B", "C", "C", "C", "C", "B", "B", "C", "A", "A"],
 ["B", "A", "C", "B", "B", "A", "B", "B", "A", "A"],
 ["B", "C", "B", "C", "A", "A", "A", "B", "C", "B"],
 ["B", "B", "B", "A", "C", "B", "A", "C", "B", "A"],
 ["A", "A", "A", "C", "A", "C", "C", "B", "A", "C"],
 ["A", "B", "B", "A", "A", "C", "B", "C", "C", "C"],
 ["C", "B", "A", "A", "C", "B", "B", "C", "A", "A"]

Output
====

The left-up and right-down positions of the largest rectangle corners:

> [1, 1, 8, 4]

Explanation:
===

A rectangle is valid if it has same "letter" on all corners. Sample rectangles:

[![enter image description here][1]][1]

This question is posted on Stack Overflow with title: [How to find the largest rectangle in a 2D array formed by four identical corners?][2] and with this rude JS solution (I can say "rude" because is my code ;) :

 var l = [ 
 ["B", "C", "C", "C", "C", "B", "B", "C", "A", "A"],
 ["B", "A", "C", "B", "B", "A", "B", "B", "A", "A"],
 ["B", "C", "B", "C", "A", "A", "A", "B", "C", "B"],
 ["B", "B", "B", "A", "C", "B", "A", "C", "B", "A"],
 ["A", "A", "A", "C", "A", "C", "C", "B", "A", "C"],
 ["A", "B", "B", "A", "A", "C", "B", "C", "C", "C"],
 ["C", "B", "A", "A", "C", "B", "B", "C", "A", "A"]];
 
 var squares = [];
 //for each position
 for (var column=0; column< l[0].length; column++) 
 for (var row=0; row< l.length ; row++ ) 
 //look for all scuares from this position:
 for (var next_column = column+1; next_column < l[0].length; next_column++ )
 if ( l[row][next_column] == l[row][column] )
 for (var next_row = row+1; next_row < l.length; next_row++)
 //if it is a square
 if (l[next_row][column] == l[row][column] && 
 l[next_row][next_column] == l[row][column]) 
 //annotate square
 squares.push( [ column, row, next_column, next_row ] );
 
 //get areas from squares
 var area_l = squares.map(function(an_square) { 
 return (an_square[2]-an_square[0])*(an_square[3]-an_square[1]);
 });
 
 //search for big area
 var max_area_index = area_l.indexOf(Math.max( ...area_l ));
 
 //here it is
 console.log( squares[max_area_index] );

Ok, is my first post, be tolerant with me please. I will change all you say to improve the quiz.

 [1]: https://i.sstatic.net/ZIkkF.png
 [2]: https://stackoverflow.com/questions/49708412/how-to-find-the-largest-rectangle-in-a-2d-array-formed-by-four-identical-corners