Okay, so first of all, let's address how to answer this question so that you are making an argument, not summarizing the book.
Making an argument: Summary would be a statement like "Ralph is better leader than Jack when he does x and x." This is considered summary because all you are doing is stating a belief and then listing things that happen. The best way to avoid summary and actually make your argument is with one simple word: because. By using "because" in a statement, you are making it into a cause-effect situation: "Ralph is a better leader than Jack because x." Now, in your argument, you must prove that this is true!
Using the evidence: The quotation you have been asked to use is from the 4th chapter. In this scene, Ralph and Piggy noticed from the beach that the fire has gone out, and they run to the signal spot. When they get there, it's been abandoned, but they are quickly joined by the "hunters", led by a triumphant Jack. Instead of commenting on the carcass that Jack is carrying, which will be food, Ralph instead flatly states "You let the fire go out" (52).
You have to use this statement to prove that Ralph is better at leading than Jack; what is significant here is the conflicting value systems between the two leaders. Jack, as a leader, has been successful in attaining a short-term goal: food. Ralph, as a leader, has been unsuccessful in attaining a long-term goal. Now, for your argument, which favors Ralph, you can compare their goals and explain to your reader that although Ralph has been unsuccessful, his goal has a wider scope and greater implications than Jack's. Although Jack has been successful, his leadership will never get the boys off of the island, and Ralph's will.