Final answer:
Ada would likely use email to respond to the customer's question, as it is direct, efficient, and provides a written record of the correspondence. Therefore, the correct option is D. email
Step-by-step explanation:
If Ada's boss has asked her to respond to a customer's question about their company's product, the type of business writing she would most likely prepare is a D. email. Email is typically the preferred method for direct and timely communication with a customer, as it allows for a personal touch and the ability to address the customer's concerns quickly and efficiently. Additionally, email communication provides a written record of correspondence, which can be useful for any future reference.