Right answer is (c) 20tqε
To explain I would say: Unless the outer edge is continuously stiffened, the outstand of stiffener from face of web should not exceed 20tqε,where tq is thickness of stiffener. When the outstands of web is between 14tqε and 20tqε, then the stiffener design should be on the basis of a core section with an outstand of 14tqε.