Which propagation method involves removing the bark and applying rooting hormone?

The propagation method that involves removing the bark and applying rooting hormone is air layering. This technique is used to encourage a plant to develop roots on a stem while it is still attached to the parent plant. By carefully removing a ring of bark from a section of the stem and applying rooting hormone at that point, the plant can stimulate root growth. After some time, roots will begin to form in the area where the bark was removed, allowing the section to be cut away and planted as a new independent plant. This method is particularly useful for propagating certain species that are difficult to root from cuttings alone.

The other methods listed are distinct and do not include the bark removal and application of rooting hormone. Simple layering involves bending a branch to the ground and covering it with soil, allowing it to root while still attached to the parent plant. Tip layering also involves bending a stem, but it focuses on the tips of branches, which are buried to induce root formation. Split-vein cuttings involve making cuts in the stem to promote rooting but do not require the removal of bark as in air layering. Thus, air layering is the only method that fits the description provided in the question.