A lens is a piece of transparent glass bounded by two spherical surfaces.

focal length is basically length from the optic centre to the focus, so it is expressed in metres

Principal axis is a straight line passing through the two centres of curvature of lens.

The principal axis is also called optical axis of a lens, passing through the optical center of the lens.

Aperture of a lens is the length of the surface of lens exposed for incident light.Focal length is the distance between focus of the lens and lens optical center.Principal focus is the focus from which a ray of light traveling parallel to optical axis passes through or appears to pass through after refraction.There is no term like efficiency of a lens.but there is magnification of a lens.

When the object is placed at infinity, i.e., when the rays are parallel to the principal axis, the value of focal length is equal to the value of the image distance.

Principal axis is also called optical axis of the mirror or lens .

The focal length is associated with distance. So, it can be expressed in metre.

The term momentum is not associated with a lens. It is the product of mass and velocity of a body.

${R _1} = 60\,cm,\,\,{R _2} = \infty ,\,\,\mu  = 1.6$

A point on the principal axis of a lens, to which light rays parallel to the principal axis converge after passing through the lens, is  called focus of the lens.

In this style of multiple-choice question, the I, II, and III questions provide you with three possible answers, and the five answer choices list different combinations of those three. Theres an upside and a downside to questions like these. Suppose you know that a concave mirror has f >0 and a convex mirror doesnt, but youre not sure about a converging lens. The downside is that you cant get the right answer for sure. The upside is that you can eliminate B, D, and E, and have a 50% chance of guessing the right answer. As long as youre not afraid to guessand you should never be afraid to guess if youve eliminated an answerthese questions shouldnt be daunting. 
The value of f for a converging lens is positive, so the answer is C . 

Centre point of a lens is called the Optical Centre of the lens. Any ray of light passing through it goes un-deviated.

$\dfrac{1}{f _r}=\dfrac{1}{f _1}+\dfrac{1}{f _2}-\dfrac{d}{f _1f _2}$