Substitutes decrease demand for motorcycles

Steps:

• Identify that a leftward demand shift means decreased demand due to non-price factors like related goods.
• Recognize cars as substitutes for motorcycles; lower car prices make cars more attractive.
• Conclude that consumers switch to cheaper cars, reducing motorcycle demand.
• Confirm this shifts the entire demand curve left, unlike price changes on the good itself.

Why A is correct:

• Cars are substitute goods; per the law of demand, a fall in substitute price decreases demand for the other good, shifting its curve left.

Why the others are wrong:

• B: Motorcycle price fall causes movement along the demand curve, not a shift.
• C: Higher motorcycle tax shifts supply curve left by raising production costs, not demand.
• D: Public transport price rise increases motorcycle demand as a substitute, shifting curve right.

Final answer: A