Concept:Two radii of a circle form an isosceles triangle with a chord, so the base angles are equal.Explanation:In △ONS, ON=OS because both are radii of the same circle.Hence, ∠ONS=∠OSN.Given ∠NOS=108∘.The sum of angles in a triangle is 180∘.So, 2∠OSN+108∘=180∘.Therefore, 2∠OSN=180∘−108∘=72∘.Thus, ∠OSN=272∘=36∘.Answer:∠OSN=36∘ (Option C).
