What term best fits "a part, segment, or sub unit of a company whose manager is accountable for specified activities?"

The term that best fits "a part, segment, or sub unit of a company whose manager is accountable for specified activities" is a responsibility center. This concept refers to any unit within a larger organization where a manager is responsible for controlling and managing specific operations, resources, and budgets.

A responsibility center can be any division or department in a company, and it serves to improve accountability by defining clear lines of authority and responsibility. The performance of such centers is typically evaluated based on certain criteria, which can include profits, revenues, or costs, depending on the type of responsibility center it is categorized into. Regardless of the specific focus, the key aspect is the accountability of the manager for the unit's performance.

While the other options like cost center, profit center, and revenue center represent specific types of responsibility centers with distinct focuses, the broader term "responsibility center" encompasses all these definitions and emphasizes the accountability aspect across various types of managerial units within a company.