Which of the following is an aspect of usability in software design?

Usability in software design primarily focuses on how easy and user-friendly a software application is for the end user. The correct choice, which emphasizes "Ease of navigation," directly correlates with this concept. When users interact with a software application, their ability to find information and perform tasks efficiently is critical. Ease of navigation ensures that users can move seamlessly through the application without confusion, thereby enhancing their overall experience and effectiveness.

Other aspects, such as system upgradability, data encryption, and network connectivity, are certainly important in the broader context of software development and security. However, they do not directly address usability. System upgradability relates to the ability to enhance or modify the software in response to new requirements. Data encryption pertains to the security measures that safeguard user data, and network connectivity refers to the software's ability to connect to the internet or networks. While these are significant features, they do not enhance the immediate user experience in terms of usability like ease of navigation does.