The main idea is found by looking at the general idea of the text you are reading, what is it about? What does it mention?
Answer: Chronological order.
Chronological order means describing the events in their order of occurrence in time. In this case, "How to jump start a car battery" describes a process that has several steps. As it is very complex, the best way to write about this process is chronologically. This would allow the readers to perform the actions as they read along, and would decrease their chances of making a mistake.
From what i remeber the answer is blue
