Which phrase is commonly used to indicate a sequence of events in writing?

The phrase "Firstly" is commonly used to indicate a sequence of events in writing. It serves as a clear signal to the reader that the writer is about to present the first point or step in a series. This structure is particularly useful for organizing thoughts and arguments in a logical manner, allowing the reader to easily follow the progression of ideas.

Using "Firstly" also helps to establish an order, which is important when discussing multiple points or steps in a process. This clarity is crucial in technical or professional writing, such as in the Occupational English Test (OET), where the audience needs to comprehend the information quickly and efficiently.

While the other phrases have their own specific uses—"As a result" indicates a consequence, "For this reason" emphasizes a justification, and "Unfortunately" expresses regret—they do not inherently signal a sequence of events. Hence, "Firstly" is the appropriate choice to indicate an orderly sequence in written communication.