What is defined as a file or set of files sent by one system to client systems, which executes software or controls applications?

Mobile code refers to programs or scripts that are transmitted across networks and executed by client systems. This type of code is often designed to perform specific tasks within an application or the operating environment on the client side. It enables execution of content on client systems, which can range from simple scripts running in web browsers to complex applications provided via platforms like Java applets or ActiveX controls.

The concept of mobile code is important in software development security as it presents potential security risks, particularly if the source of the code is untrusted. Proper security measures, including sandboxing, validation, and restricting permissions, are critical to mitigate these risks.

Other terms in the options describe different aspects of computing. Executable content typically refers to any file type that can be run as a program but does not specifically denote the transmission aspect inherent in mobile code. A data package generally conveys a bundle of information or files rather than the functionality associated with executing applications. Lastly, a system call refers to an interface that allows a program to request services from the operating system, which is distinctly different from the concept of code being sent and executed on a client system.