What is the primary purpose of UML (Unified Modeling Language) in object-oriented systems?

The primary purpose of UML (Unified Modeling Language) in object-oriented systems is to visualize object-oriented systems. UML provides a standardized way to represent and model the structure and behavior of a system, using various diagram types such as class diagrams, use case diagrams, sequence diagrams, and more. This visualization helps stakeholders, including developers, designers, and clients, to understand the relationships and interactions between objects within the system. By creating a clear visual representation, UML assists in the design process, facilitates communication among team members, and helps ensure that the system meets the specified requirements.

The other options focus on aspects unrelated to UML's fundamental objective. For instance, defining programming syntax pertains more to programming languages than to modeling tools like UML. Optimizing code performance is an operational concern that deals with the efficiency of the code rather than its design representation. Managing project timelines falls under project management rather than the modeling of system architecture, which is where UML excels.