The correct option is A Protoderm
The external protective layer of dicot root is called as epiblema. Epiblema is derived from protoderm. They are specially adapted for absorbing liquids. It is uniseriate. Epiblema is formed by a single layer of compactly arranged parenchyma cells. Cells are thin walled. Some of these cells extend into unicellular root hairs.
Thus, the correct answer is option A.