The correct option is A Increase of sugar concentration in phloem. In contrast to xylem transport, the mass flow mode of phloem transport requires that the plasma membrane remain intact. At the source end of the phloem (area where sugar is synthesized), sugars are moved into the phloem sieve elements. This increase in solute decreases the water potential of the cell and causes water to flow in from surrounding areas by osmosis. The increase in the volume of water in the cell causes an increase in pressure, which forces the sugar/water/amino acid solution to move toward the sink tissue.
At the sink tissue, the sugars are off-loaded which increases the water potential and causes water to flow out of the phloem by osmosis. The sieve elements must remain at least partially alive and keep a functioning plasma membrane in order to help control the flow of sugars into and out of the sieve element.