If a person wanted to stimulate root growth on a plant cutting, he could apply synthetic to the stem.
a. abscisic acid
b. ethylene
c. gibberellin
d. auxin
e. strigolactone

Auxin

Explanation:

Plant hormones are small organic molecules,which serve as specific chemical signals between cells.

    *Auxins* are group of substances characterized by their capacity to induce elongation in shoot cells they do this by stimulating the enlargement of cells through the irreversible stretching of cell walls.These Auxins have structures similar to the amino acid trytophan and occur in the plant and animal cells.They are mainly produced at the apical meristems,bud and young leaves.*Their main function is to stimulate stem elongation and root growth*.

 So,when a person wants to stimulate root growth on a plant cutting,he could apply synthetic auxin to the stem inorder to instigate the growth of that root.