(a) Less than 10 cm
A concave mirror forms a virtual, erect and enlarged image only when the object is placed between the pole (P) and the principal focus (F), i.e., less than the focal length (10 cm) from the mirror.
From Table 9.1 (Image formation by a concave mirror), when the object is placed between P and F, the image formed is virtual, erect, and enlarged — the only case where a concave mirror gives an erect image. Since f = 10 cm, the object must be placed at a distance less than 10 cm. Students often confuse this with convex mirrors; remember, a concave mirror gives an erect image only when the object is within the focal length.