Answer: Mouth to mouth advertising.
Explanation: Have you ever heard that the best advertising is the one that goes from mouth to mouht. Well basically every company and enterprise is trying to fulfill these "rule" it´s like the basis of advertisement. When you show that your product or service is way much better that the competition then people will surely talk about it. But you have to PROOF to your clients that you´re the best amog other inr order for them to buy your product/service.