What action deepens a product line's depth?

Adding a product to a product line deepens its depth because it involves expanding the range of options available within a specific category. Product line depth refers to the number of variations or items within a specific product line, such as different flavors, sizes, or models. By introducing a new product, a company enhances its offerings and provides more choices for consumers, which can lead to increased customer satisfaction, better addressing consumer preferences, and potentially higher sales.

In contrast, removing a product would decrease the depth of the product line, as it reduces the number of available options. Redesigning existing products may improve their appeal or functionality but does not necessarily add new options to the line. Increasing advertising efforts can enhance visibility and drive sales for existing products, but it does not affect the actual number of products available in that line. Therefore, the action that specifically deepens a product line's depth is indeed the addition of a new product.