Final answer:
Option C, a report with details about a company's marketing strategy, is an example of an informational business message. It serves to inform relevant parties about the company's marketing initiatives.
Step-by-step explanation:
An example of an informational business message is C. A report with details about a company's marketing strategy. This type of document is designed to inform employees, stakeholders, or other interested parties about specific aspects, changes, or methodologies within the company's marketing efforts. On the other hand, a letter requesting a speaker (option A), a proposal to purchase a product (option B), or a memo urging professional etiquette (option D) are all persuasive or directive in nature and fall under different categories of business communications.