The correct option is B Venation
Venation is arrangement of veins in leaves. Venation is a distinguishing character between monocots and dicots. Aestivation is arrangement of sepals and petals in flower buds before it opens and placentation is defined as the arrangement of the placenta in the ovary of a flower. The placenta connects the ovules with the wall of the ovary. Aestivation and placentation are not the distinguising feature of monocots and dicots.
A monocot can be distinguished from a dicot by venation. In monocots, there are usually a number of major leaf veins which run parallel the length of the leaf. It shows parallel venation. In dicots, there are usually numerous auxiliary veins, which reticulate between the major ones. It shows reticulate venation.
Thus, the correct answer is option B.