Answer: D) It creates dramatic tension.
Step-by-step explanation: In the given passage we can see the description of how the character was about to cry, but he/she was trying very hard to keep smiling. From the given options, the one that represents how the difference between the character's spoken words and internal thoughts affect the story, is the corresponding to option D: it creates dramatic tension, because the reader doesn't know if the character will be able to mantain the smile, of if he/she is going to start crying.