Answer:
(b). Focal length
Step-by-step explanation:
Focal length :
The focal length is the distance between the focal point and center of the optical system.
The focal length is positive for convex lens and convex lens is converging lens.
The focal length is negative for concave lens and concave lens is diverging lens.
When the light rays will be focused to a single point . That point is called focal point.
Hence, The distance from the focal point to the optical center of the lens is called focal length.