Maria, the marketing manager at Jack and Jill, wants to optimize their campaigns based on the actions of first-time customers. Which of the following metrics should they use?

New-to-brand (NTB) is the correct metric because it specifically measures purchases made by customers who haven't previously bought from the brand on Amazon in the last 12 months. This directly addresses Maria's goal of optimizing campaigns based on the actions of first-time customers. Click-through rate (CTR) measures ad engagement, not purchase behavior or customer type. Spend indicates the total amount invested, but not the customer's newness. Cost-per-click (CPC) measures the efficiency of ad clicks, not the acquisition of new customers.