Final answer:
The marketing department is considering the Promotion element of the marketing mix, which involves the use of different advertising strategies to communicate with target customers and stimulate sales.
Step-by-step explanation:
When the marketing department at a baby store collaborates with an advertising agency to decide on the type of advertising, as well as where and when ads should be placed to make the most impact, they are considering the Promotion aspect of the marketing mix. Promotion involves communicating the benefits and features of a product or service to target customers to stimulate sales. Successful promotional strategies often include a mix of advertising methods tailored to the product's target audience and the media consumption habits of that audience.