Distance between principal focus and optical center of the lens is
A. Diameter
B. Focal length
C. Principal axis
D. Optical center

Hint We will draw figures to find out the distance. We need to find where the point of principal focus and optical center. We will observe that the length between their two points is known as focal length.

Complete step by step solution:



Let us take an example of biconvex lens, to understand this
Biconvex lens: It is also known as converging lens. It is formed by joining 2 convex lenses together.
Focal length: distance lying between principal focus and optical center.

Principal focus - It is a point where all the parallel lines meet or are focused.

Principal axis - The line principal focus passing through the optical center is known as
principal axis.

Optical center - Parallel ray coming from an object strike at a certain point after which it gets refracted, that point is known as optical center. This crosses the principal axis also.

Diameter - It is a parameter which is inversely proportional to focal length. The distance between topmost and the bottom of the lens is known as the diameter of the lens. It is shown in figure smaller the diameter large will be focal length and vice versa. It is denoted by D.

Note
We have found that option (c) is about some axis and option (d) is about center. So, they cannot be regarded as the distance of something. Some tricky part in the question that one can go wrong is between diameter and focal length. Another method to understand is through practical observation in labs.