What is exclusive for angiosperms ?

A. vessel element i.e. trachea is one of the cell types found in the xylem which is the water-conducting tissue of plants.

B.  Secondary growth is characterized by an increase in thickness or girth of the plant.

C. The angiosperms are vascular seed plants and there the ovule (egg) is fertilized and develops into a seed enclosed hollow ovary. Double fertilization is main feature of angiosperms. It also involves the combination of a female gametophyte with two male gametes.

D. Autotrophic nature means some plants producing their own food.

Thus the answer is double fertilization i.e. C.