Verified answer

You can easily see, if you had 1 mole of SiO2 and 3 moles of C, it would just use up all of both reactants, they are asking, what would happen if you had 4 moles of each, you can easily see there will be a lot of the SiO2 left over.

assume one of the reactants is consumed completely, that would be carbon. because 4 moles of carbon can only react with 4/3 mole of SiO2. therefore SiO2 is in excess.