Answer:
b. Suggestion 2
Explanation:
Suggestion 2 will increase the demand for public transportation because private transportation is a substitute. If it is expensier to use private transportation, some people that before used private transportation will start using the public one. Suggestion 1 and 3 will not increase demand (shift the demand curve in the demand and supply graph), they will result in changes in the quantity demanded (movements along the demand curve).