The marks (out of 25) obtained by students of a certain class are 11, 12, 14, 13, 17, 11, 12, 12, 17, 21, 24, 14, 11, 13, 12, 17, 21, 19, 19, 24, 14, 15, 12, 17, 15, 19, 21, 13, 16, and 15.
What is the mode of marks obtained by students of the class?