COKE IS IT: That many in fashion have a preference for Diet Coke is hardly a secret — among them Karl Lagerfeld, who seems to have a perennial thirst for the soft drink. Now the association to fashion deepens. As part of a recent can redesign, Diet Coke has unveiled a T-shirt by Gustavo Alonso, a Miami International University of Art & Design student who participated in the Diet Coke Young Designer Challenge (a partnership with the Council of Fashion Designers of America). His design — featuring an illustration of a woman in a high-collared red coat cinched at the waist — was selected by judges Cynthia Rowley, Zac Posen and L’Wren Scott. The T-shirt will be available at Target from Sept. 3.