Byron has ended a sales message with the sentence Book your vacation by May 15 to receive our reduced rate. What technique is he using to motivate action?

There are a variety of techniques employed by the marketing associates to motivate the consumer's action in the minimum possible time that ensures the reach of their product to the widest audience effectively. They conduct a market research in order to analyze the trends and customer's behavior in order to shape/develop their marketing or advertising strategies accordingly.

In the given sales message, the 'setting of deadline' would encourage the customers to 'book their vacations prior to 15th May' as they want to grab the deal at their earliest. This deadline is a strategy that compels the audience to react within a limited time and ensure the positive outcomes(higher sales due to increased number of customers) for the company. Thus, 'setting a deadline' is the answer.