In the following

We know that, tangent through a circle is perpendicular to the radius through point of contact.

⇒ OP ⊥ XY and ΔOPQ is a right-angled triangle

⇒ OP² + PQ² = OQ²

⇒ r² + 12² = 13² [∵ OP = radius (r)]

⇒ r² = 169 – 144 = 25 ⇒ r = 5 cm

Hence, Option B is correct.