The guiding principle Fed and Guido followed is_____.

　　定位到文中第三段的第四句，“The only rule they had when they started out was to stick to their guiding principle-to try to create the world's best gelato.”， 他们开始时唯一的规则是坚持他们的指导原则——努力创造世界上最好的意式冰淇淋。故选C。