How does Ralph represent common sense?

Ralph is the one who insists upon keeping the fire going. He knows this is their best chance to be spotted and potentially rescued. Ralph also is the one who keeps reminding the other boys the importance of building the shelters. Only Piggy and Simon fully support him in these two necessities. 

Ralph stands in contrast to Jack who is more interested in hunting. There is the logical (common sense) fact that having meat is...

Ralph is the one who insists upon keeping the fire going. He knows this is their best chance to be spotted and potentially rescued. Ralph also is the one who keeps reminding the other boys the importance of building the shelters. Only Piggy and Simon fully support him in these two necessities. 

Ralph stands in contrast to Jack who is more interested in hunting. There is the logical (common sense) fact that having meat is a necessity (even though the boys have plenty of fruit). But Jack's interest in hunting stems from a primal desire for violence. Thus, Jack regresses and becomes more savage. In contrast, Ralph sticks with his common sense and his sense of responsibility. Piggy also represents and illustrates common sense and reason. Simon is wise. Piggy is logical. Ralph is responsible. They are the civilized leaders of the island, with Ralph being the alpha leader of this group. 

In Chapter 4, Jack and his group are out hunting. They neglect the fire and it goes out. Unfortunately, a ship passes while the fire is out. With no fire, there is no smoke. With no smoke, there is no signal to attract the attention of a passing ship. Here, we see the dichotomy between Ralph and Jack. Jack thinks they needed to go hunting. Ralph is furious that they let the fire go out. Clearly, Ralph shows basic common sense in confronting Jack. Ralph repeats "You let the fire go out." He repeats, "There was a ship." He is being simple and direct as a way to best communicate his frustration.