The mention of molten fat wasn't to indicate that the early man used it to make his lamps 
He simply got inspiration from the droppings of molten fat 
The correct answer should be sea shells because it clearly states that he used hollow sea shells to make his lamps

Read the passage below carefully and answer the question that follows:Primitive man was probably more concerned with fire as a source of warmth and as a means of cooking food than as a source of light. Before he discovered less laborious ways of making fire, he had to preserve it and whenever he set out on a journey, he carried a firebrand with him. His discovery that the firebrand, from which the torch may well have developed, could be used for illumination was probably incidental to the primary purpose of preserving a flame. Lamps, too, probably developed by accident. Early man may have had his first conception of a lamp while watching a twig or fibre burning in the molten fat dropped from a roasting carcass. All he had to do was to fashion a vessel to contain fat and float a lighted reed in it. Such lamps made of hollowed stones or seashells, have persisted in identical form up to quite recent times.<hr/>Which named lamp container was made from a marine animal’s hard outer covering, rather than from stone or described only by the general word vessel?

Earlier discussion