Which distance is equal to the focal length of a lens?
A. The distance between a distant object and its image.
B. The distance between the image of a close object and the centre of the lens.
C. The distance between the image of a distant object and the centre of the lens.
D. The distance between two principal foci.
Solution: Option C.
Recall that only reflected rays from distant object (object placed far away) are considered parallel. When these parallel light rays pass through the converging lens, they converge to a point, Focal Point (F).
Only then, the distance between the (sharp) image of distant object to the centre of the lens, optical centre (C), is known as focal length (f).
Refer to the diagrams below.
Also refer to another question on lens: http://evantoh.posterous.com/2010/11/09/converging-lens-important-concepts