Group O, also known as the noble gases, are unreactive because their highest occupied energy level is full, which means they have a stable electron configuration. They have no tendency to gain, lose or share electrons, as they already have the full complement of electrons in their outermost shell. This makes them extremely unreactive and inert, which is why they are often referred to as noble or inert gases. Therefore, the correct answer is "their highest occupied energy level is full".