According to the acid-growth hypothesis, auxin works . dissolving sieve plates, permitting more rapid transport of nutrients. b. dissolving cell membranes temporarily, permitting cells that were on the verge of dividing divide more rapidly. c. changing the ph within the cell, permitting the electron transport chain to operate more efficiently. d. increasing the activity of proton pumps that lower the ph of cell walls, allowing cells to elongate.

The correct answer is D increasing the activity of protons pumps that lowers the pH  of the cell walls allowing cells to elongate.

Explanation:

The plant growth hormone auxin work by increasing the H+ ion concentration thus decreasing the pH of the apoplast.

    The decrease in the pH of the apoplast helps in the cell wall extension that ultimately result in cell growth.

   Auxin hormone exert this effect by increasing the activity of increasing the activity of proton pumps in the apoplastic membrane.