Which of the following is not an action of ethylene ?

The correct option is B Inhibits root hair growth – decreasing the efficiency of water and mineral absorption
Ethylene:
Ethylene is the only gaseous PGR and is effective in conc.$0.01$ to $10$ ppm.
It is produced by most or all plant organs but maximum production occurs in ripening fruits and senescent leaves.

Role of ethylene:

• Root initiation: Low concentration of ethylene induces rooting, growth of lateral roots and root hairs.
• Inhibition of stem elongation and stimulation of transverse growth. Ethylene causes increase in the girth of the plant, and promotes horizontal growth.
• Senescence: It increases senescence of leaves and flowers.
• Abscission: It induces abscission of leaves, flowers and fruits.

Hence, option B is correct.