Answer:
b. reminder
Explanation:
Based on the information provided within the question it can be said that Crest's advertising would be classified as a reminder. This is a follow up to an existing advertisement campaign with the goal of bringing more attention to the product or service and remind people to buy it, usually also introducing a new modification to the product/service. Similar to what Crest is doing by letting people know about the advantages and reminding the customers about the newest flavor.