A user's smartphone data usage is well above average.

A firewall is a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. In this scenario, the user is concerned about an installed application transmitting data in the background, so a firewall would be the best solution to address their concern. By installing and configuring a firewall, the user can block unauthorized connections to and from the device, and receive alerts whenever an application tries to access the internet.