Answer : The number of moles of sulfur needed are, 15 moles
Explanation : Given,
Moles of sulfur dioxide = 15 mole
The balanced chemical reaction will be,

By the stoichiometry we can say that, 1 mole of sulfur react with 1 mole of oxygen gas to give 1 mole of sulfur dioxide as a product.
From the balanced chemical reaction we conclude that,
As, 1 mole of sulfur dioxide produces from 1 mole of sulfur
So, 15 moles of sulfur dioxide produces from 15 moles of sulfur
Therefore, the number of moles of sulfur needed are, 15 moles