In Martina's role as an administrative professional, the type of business letter that is most likely to be requested by her manager is an acceptance letter. This is a common type of correspondence used in professional settings to formally acknowledge the acceptance of an offer, such as a job offer, proposal, or agreement.
The other options, such as a "Dear John letter" (typically used to break off a romantic relationship), a "happy birthday letter" (which is more personal than professional), and a "chain letter" (often considered spam), are less relevant in a business context.