The correct option is D Mesophyll tissues are undifferentiated.
Mesophyll tissues are undifferentiated in monocot leaves. Monocot leaves have veins that are parallel to one another through the length of the leaf and do not branch. There may or may not be a mid-vein present but all other veins are of a single order. Stomata can be found on both leaf surfaces. The palisade layer is indistinct from the spongy mesophyll layer in monocots. Cells in both layers are roughly isodiametric.
Thus, the correct answer is option D.