The correct option is A Die naturlichen Pflanzenfamilien
Adolf Engler, was a German botanist, who is known for his work on plant taxonomy and phytogeography, such as 'Die naturlichen Pflanzenfamilien' (The Natural Plant Families), that was jointly edited by Karl von Prantl , comprising of 23 volumes between years 1887-1915 . Even now, his system of plant classification, the Engler system, is widely used by many herbaria and is followed by writers of many manuals and floras.
So, answer is- 'Die naturlichen Pflanzenfamilien'.