The set of natural numbers N is partitioned into arrays of rows and columns in the form of matrices as M1 = [1], M2 = [(2, 3), (4, 5)], M3 = [(6, 7, 8), (9, 10, 11),(12, 13, 14)], ....., Mn = [...] and so on. Find the sun of the elements of the diagonal in M₆
(a) 144
(b) 441
(c) 321
(d) 461