A student determines the focal length of a device X, by focusing the image of a far off object on the screen positioned as shown in the figure. The device X is

Question

$\begin{array}{1 1} (a)\; \text{convex lens} \\ (b)\; \text{concave lens} \\ (c)\; \text{convex mirror} \\ (d)\; \text{concave mirror} \end{array}$