Phloem loading from a sink increases solute concentration Steps:

• Carbohydrates unloaded from sink enter phloem sieve tube, raising solute concentration inside.
• Higher solute concentration lowers water potential (more negative) in sieve tube.
• Lower water potential draws water in from xylem via osmosis.
• Inflow of water increases turgor pressure and volume of liquid in sieve tube.

Why D is correct:

• Adding solutes decreases water potential per solute potential formula (ψ_s = -iCRT); water influx expands volume to maintain pressure-flow.

Why the others are wrong:

• A: Water potential rises (not lowers) with solute addition; volume increases (not decreases).
• B: Volume decreases (not increases) if water potential rose, but solute addition lowers it.
• C: Volume increases (not decreases) due to osmotic water entry.

Final answer: D