The upper layer of Zea mays is Adaxial epidermis and the lower layer is called the Abaxial epidermis.
In the vascular bundle, the larger vessels are the xylem and the small vessels are the phloem.
Thus, A-Adaxial epidermis, B-Abaxial epidermis, C-Xylem, D-Phloem.
So, the correct answer is (B).