Final answer:
To achieve a magnification of -1 with a positive lens, a real object must be placed at a distance of 2f from the lens, where f is the focal length of the lens. This will produce a real, inverted image of the same size as the object.
Step-by-step explanation:
The question is asking about the conditions required to achieve a magnification of -1 with a positive lens (a converging lens). Magnification (m) is calculated as the image distance (di) divided by object distance (do), and a magnification of -1 denotes that the image is real, inverted, and of the same size as the object. For a positive lens, this condition is met when the object is placed at a distance of 2f from the lens, where f is the focal length of the lens. At this distance, the rays from the object would converge to form an image on the opposite side of the lens at a distance of 2f, creating a real, inverted image of the same size.
Based on the information and the physical principles of lens behavior, the correct answer is to place a real object at a distance 2f from the lens. Placing the object at the focal point would result in no real image formation as the rays would be parallel to each other after emerging from the lens. A virtual object or any position resulting in a virtual image would produce magnification greater than 1 and a positive magnification, not fulfilling the condition for -1 magnification. Therefore, options involving virtual objects or images are incorrect.