Lignin's structural support and waterproofing in xylem

Steps:

• Identify lignin's key roles: strengthens cell walls for rigidity and resists water flow to maintain tension in tall trees.
• Match properties: 1 (hydrophobic, prevents leakage), 2 (mechanical strength, withstands pressure), 3 (rigidity, supports vessel shape), 4 (degradable, irrelevant to function).
• Eliminate options excluding essential properties or including irrelevant ones.
• Select combination covering hydrophobicity, strength, and rigidity for efficient water transport.

Why A is correct:

• Properties 1, 2, and 3 align with lignin's definition as a polymer reinforcing xylem walls against collapse and evaporation under high tension (cohesion-tension theory).

Why the others are wrong:

• B includes 4 (degradability), which hinders long-term vessel function.
• C omits 2 (strength), risking vessel collapse in tall plants.
• D excludes 1 and 3, ignoring waterproofing and rigidity needs.

Final answer: A