Focal length of lens = 25 cm (rays from sun converge at focus). For a virtual image in a convex lens, object must be placed between F and optical centre, i.e., at a distance less than 25 cm → (A) 20 cm.
Sunlight (parallel rays) converges at 25 cm → f = 25 cm. A convex lens forms a virtual, erect, magnified image only when the object is placed between the optical centre and F (u < f). Among the options, only 20 cm < 25 cm satisfies this condition. All other options (40, 50, >50 cm) place the object beyond F, giving a real image.