I need to know the key "action/operation" and the smallest time complexity for the question above. Thanks

Questions (15 points): a) For each of the following problems, propose the key "action/operation" you would use to determine the time complexity (short answers are fine here). What is the smallest time complexity of an efficient algorithm solving that problem? a. (3 points) Consider an nxn matrice B(by) where i andj vary from 1 ton. The problem is to compute the trace of Matrix B i.e., adding the elements of the matrix diagonal. Key action/operation is smallest time complexity is