The correct answer is option 3, a convex lens of small focal length.

When reading very small letters printed on a label, we usually require a magnifying glass or lens to make the letters appear larger and clearer. A convex lens is used for magnification as it converges light rays, making objects appear larger. The focal length of a lens determines its ability to magnify objects.

Option 1, a convex lens of large focal length, would not be ideal for reading very small letters. A lens with a large focal length would not converge the light rays enough, resulting in minimal magnification.

Option 2, a concave lens of large focal length, would actually diverge light rays, causing the letters to appear even smaller and harder to read.

Option 4, a concave lens of small focal length, would also not be suitable for reading small letters. A concave lens diverges light rays, resulting in a reduction in size and clarity.

Therefore, option 3, a convex lens of small focal length, is the best choice for reading very small letters on a label. It will converge the light rays and provide magnification, making the letters easier to read.