Sherlock Holmes is using an 9.99-cm-focal-length lens as his magnifying glass. To obtain maximum magnification, where must the object be placed (assume a normal eye)?

The object must be placed just ahead of focus towards the lens.

Explanation:

As we know that a magnifying glass is a convex lens and for the convex lens and for a convex lens we get the image virtual, erect and magnified only when it is placed between the optical center and the focus of the lens.

In that range when the object is nearest to the focus of the lens it has the maximum size.

So it can be possible if the object is placed at 9.98 cm from the optical center.

The largest image is formed at infinity when the object is placed at focus but it is not visible to the normal eyes.