What type of solution should Helen use to prevent staff from sideloading applications on their Android devices?

Using a Mobile Device Management (MDM) solution is the correct approach for preventing staff from sideloading applications on their Android devices. MDM solutions are specifically designed for managing mobile devices within an organization. They enable IT administrators to enforce security policies, manage app installations, and control device settings effectively.

By implementing an MDM solution, Helen can set specific policies that restrict the ability to install applications from unknown sources, thereby preventing sideloading, which is the process of installing apps from sources outside of the official app store. This not only helps in controlling what applications are on the devices but also mitigates the risk of malware and other security vulnerabilities that may come from unverified apps.

The other options, while valuable in certain contexts, do not specifically address the issue of sideloading. Antivirus software is important for detecting threats but does not inherently prevent unauthorized app installations. A firewall can protect the network by controlling traffic but does not manage mobile device configurations. Restricting internet access may limit the ability to download apps but is not a practical or effective solution for managing device usage as a whole.