Power and focal length related, by which relation?

Linear magnification of a concave lens is always less than $1$. Why?

The linear magnification of a convex lens may be positive or negative, why?

A student obtained a blurred image of an object on a screen using a concave mirror. In order to obtain a sharp image on the screen, he will have to shift the mirror -

Four students $A\mathit{,}\mathit{ }B\mathit{,}\mathit{ }C\mathit{ }$and $D$ carried out measurements of focal length of a concave mirror as shown in the four diagrams.

the best result will be obtained by students:-

Parallel rays, from the top of a distant object, incident on a concave mirror, form an image on the screen.

The diagram correctly showing the image of the object on the screen is-