The job assigned to the narrator in the law firm was to serve summons.

Explanation:
- The job of serving summons involves delivering legal documents to individuals involved in a lawsuit, informing them of the legal action being taken against them and requiring their presence in court.
- Serving summons is an important task in the legal process as it ensures that the parties involved are aware of the legal proceedings and have an opportunity to respond.
- The role of the narrator in the law firm was specifically focused on serving summons, which suggests that they were responsible for locating and delivering these documents to the relevant individuals.
- This job requires attention to detail, organization, and the ability to communicate effectively with various parties involved in the legal process.
- Serving summons may involve visiting different locations, such as residences or workplaces, in order to deliver the documents to the intended recipients.
- The narrator's job may have also involved handling any necessary paperwork associated with serving summons, such as documenting the date and time of delivery, and providing proof that the documents were properly served.
- While the other options mentioned in the question (preparing legal briefs and fighting cases) are also common tasks in a law firm, the given information specifically states that the narrator's job was to serve summons, making option A the correct answer.
- It is important to note that different individuals in a law firm may have different roles and responsibilities depending on their expertise and position within the organization.