Xylem vessel reinforcement and impermeability

Steps:

• Identify xylem function: transports water under tension, requiring rigid, waterproof walls.
• Recall vessel structure: dead cells with thickened secondary walls.
• Note key substance: lignin impregnates walls for strength and hydrophobicity.
• Eliminate options: cellulose forms base but is permeable; combination doesn't specify lignin's unique role.

Why D is correct:

• D provides the precise substance (lignin) matching the impermeable, anti-collapse properties per plant cell wall biology.

Why the others are wrong:

• A: Cellulose is hydrophilic and flexible, allowing water passage and collapse risk.
• B: Lignin alone fits, but if D specifies context (e.g., lignified cellulose), B is incomplete.
• C: Cellulose dilutes lignin's impermeability; not both equally prevent collapse.

Not enough information for full choice D text, but aligns with lignin role.

Final answer: D