Answer:
B. If the price of gasoline rises, a smaller quantity of it will be bought.
Explanation:
A positive economic statement is the one which is based on the facts and it can be verified based on different facts established, or studies.
Whereas, a normative economic statement is the one which focuses on providing a suggestion, or an opinion of an economists for the economy, and cannot be verified.
Since statement B clearly states the fact related to the pricing policy of gasoline, and the effect which can be verified, thus, it is positive economic statement.