Answer: (C) 7 cm
The radius is perpendicular to the tangent at the point of contact, so the radius r, the tangent segment and the distance from the centre form a right-angled triangle:
\[r^2 = 12.5^2 - 12^2 = 156.25 - 144 = 12.25\] \[r = 3.5\ \text{cm}\]Diameter = 2 × 3.5 = 7 cm.