Which process involves slitting the veins on the lower surface of a leaf?

The process that involves slitting the veins on the lower surface of a leaf is known as split-vein cuttings. This technique is particularly effective for propagating certain plants, as making incisions in the leaf veins can enhance rooting by encouraging the plant to produce new roots from the cut sections. It takes advantage of the plant's natural healing mechanisms, prompting it to focus energy on developing new growth.

In contrast, the other methods involve different propagation techniques. Air layering involves creating roots on a branch while it is still attached to the parent plant, often by covering an area of the bark with a growing medium and plastic to facilitate root formation. Tip layering, on the other hand, involves burying a stem's tip into the soil while it remains attached to the parent plant, promoting rooting along the buried length. Lastly, simple layering refers to bending a stem down to the ground so that it can root while still connected to the parent plant, rather than using cuts directly on the leaf veins. These differences highlight why split-vein cuttings is the appropriate process described in the question.