Which transitional expression indicates a summary in writing?

The transitional expression "in brief" effectively signals a summary in writing because it suggests a concise restatement of previously discussed information. This term cues the reader to expect a synthesis of ideas, where the most critical points are encapsulated in a succinct manner, enabling easier comprehension and retention of the material.

In contrast, other options serve different functions in writing. For example, "moreover" is commonly used to add information or enhance an argument rather than summarize it. "Firstly" indicates the beginning of a sequence, suggesting that further details will follow, and "consequently" implies a cause-and-effect relationship, indicating the results of prior statements rather than summarizing them. Hence, "in brief" distinctly serves the purpose of summarizing content, making it the appropriate choice for this question.