Fast food is convenient because it is cheap and it’s prepared ahead of time and you don’t need to cook it. You just order and enjoy the meal especially if you don’t know how to cook. It saves time and energy.
The option that best describes the cause-and-effect structure in a memoir is when the structure focuses on an incident and its consequences. I would say that the correct option is the third one. The cause is the incident, and the effect are its consequences.