诚恳 (chéng kěn) describes a sincere and honest attitude expressed in one's words and behavior—it often applies to the tone or manner of an apology, criticism, or statement. Compared to 真诚 (zhēn chéng, genuine from the heart), 诚恳 is more about outward conduct and straightforwardness; 恳切 (kěn qiè) adds a sense of urgency and earnestness, while 诚挚 (chéng zhì) is more formal, typically used in written correspondence.