(c) At twice the focal length of the lens.
When the object is placed at 2f, the image formed is real, inverted, and of the same size as the object, also at 2f on the other side.
This is a standard ray-optics result for a convex lens. At u = 2f, magnification m = v/u = 1, giving an equal-sized image. Exercise Q.3 in the source directly confirms option (b) "At twice the focal length" as the correct answer. Don't confuse this with the principal focus (which gives an image at infinity).