Which of the following diagrams involves a virtual image ?

The third diagram

Explanation:

A virtual image is an image that can not be formed on a screen. A convex lens can form both virtual and real image depending on the position of the object from the lens. A virtual image in convex lens is formed when the object is placed between the focus and the optical center of the lens. In the third diagram, a virtual image is formed because the position of the object is between the focus and the optical center of the convex lens.