The correct answer is: A. interface
An interface is a channel for communicating across a boundary between two or more sub-systems. It is a shared boundary between two or more components that allows them to communicate with each other. The interface defines the data that can be exchanged between the components, as well as the rules for how the data is exchanged.
An actigram is a graphical representation of a system’s activities. It is used to model the system’s behavior and to identify the system’s components and their interactions.
A walk-through is a technique used to test a system by walking through the system’s steps and verifying that the system behaves as expected.
A data path is a path through a system that data follows as it is processed. It is a physical or logical path that connects the system’s components.
None of the above options are channels for communicating across a boundary between two or more sub-systems.