Auxin-driven upward bending in stem negative gravitropism

Steps:

• Negative gravitropism causes stems to grow upward, away from gravity.
• Auxin transports downward from the tip and redistributes to the lower side under gravity.
• In stems, auxin stimulates cell elongation on the side with higher concentration.
• More elongation on the lower side bends the stem upward.

Why A is correct:

• Matches the mechanism where auxin accumulation on the lower side promotes elongation there, curving the stem upward away from gravity, per the auxin growth promotion rule in shoots.

Why the others are wrong:

• B: Auxin moves down, not up, the stem.
• C: Describes phototropism (light response), not gravitropism.
• D: Incorrectly specifies upward auxin movement and confuses with phototropism.

Final answer: A