15. Given function trace defined below note: ones is a built-in function. ones(n) returns a n by n matrix with components of 1. a = function a = trace (A) [lengthx, lengthy] = size (A); 0; for i = 1: lengthx a + A(i, i); end end a What is the value of a if you execute the following code A = ones (15) *2; a trace (A) = (a) a = 15 (b) a = 30 a = 60 (d) MATLAB returns an error (e) None of above options are correct