The focal length and radius of curvature are related by the formula

The correct option is C

$r=2f$

1. Focal length: The distance between the pole (P) and focus (F) is called the focal length (f).
2. It is denoted by f.
3. If f is the focal length and r is the radius of curvature, then the relation is given by,

$r=2f$

or,

$f=\frac{r}{2}$

Hence, option C is correct.