Which of the following focuses on the simplicity and effectiveness of user interactions?

The focus on the simplicity and effectiveness of user interactions is best captured by User Interface (UI) design. User Interface design specifically pertains to the layout and interactive elements that users engage with when using a product or service. This includes buttons, menus, icons, and other elements that directly affect how a user interacts with the interface. A well-designed UI aims to enhance usability by ensuring that interactions are intuitive, efficient, and enjoyable.

User Interaction Design, while it shares similarities with UI design, often encompasses a broader scope that includes not just visual elements but also the overall user experience, including how users achieve their goals through interaction. Visual Design concentrates more on the aesthetic aspects of the interface rather than on the effectiveness of interactions themselves. Experience Design takes a step back even further to focus on the overall journey of the user with a product or service, including emotional responses and all touches points, not just the interaction interface.

In summary, User Interface (UI) design is particularly concerned with making interactions simple and effective, which aligns precisely with the question's focus.