Epidermal cells in the plant roots increase their absorption surface by developing specialized structures called root hairs. Root hairs are tiny, finger-like projections that extend from the surface of the epidermal cells. These root hairs greatly increase the surface area of the root, allowing for more absorption of water and nutrients from the soil.

Option 1: Scales are not the correct answer. Scales are protective structures found on the surface of leaves or stems, but they do not contribute to the increase in absorption surface in plant roots.

Option 3: The endodermis is a layer of cells found in the root cortex, but it does not directly contribute to the increase in absorption surface. Its main function is to control the movement of water and minerals into the root through the Casparian strip.

Option 4: Stomata are small openings found on the surface of leaves that regulate the exchange of gases. They are not present in the epidermal cells of plant roots and do not contribute to increased absorption surface.

Therefore, the correct answer is option 2: Root hairs. Root hairs are specialized structures that greatly increase the absorption surface of plant roots, allowing for more efficient uptake of water and nutrients from the soil.