Increase in quantity demanded due to lower fare

Steps:

• Initial fare OP intersects demand curve at point T, giving quantity OS.
• Reduced fare OP minus PW intersects demand curve at point R, giving larger quantity OU.
• Extra journeys equal quantity increase from OS to OU.
• Area URST spans points U (at OU), R, S (at OS), and T, with base US measuring this increase.

Why D is correct:

• URST's base US equals the increase in quantity demanded, per the demand curve definition relating price to quantity.

Why the others are wrong:

• A. VT: Small unrelated area, possibly a point or minor segment not spanning quantity change.
• B. PW: Vertical distance of price reduction, measuring fare drop, not extra quantity.
• C. RST: Triangle above new fare line, measuring consumer surplus on extra units, not their number.

Final answer: D