Use .NET Framework 4.8.1 . It remains supported as a component of the Windows OS.
Since there is no official 4.9 release, the official .NET Framework 4.8.1 download is the correct choice for users needing the latest "4.x" features. .NET Framework 4.9 Offline Installer for Windows
Below is a guide on how to handle your installation needs for the latest supported versions on Windows. Understanding the Versioning .NET Framework 4.9 Offline Installer for Windows