I downloaded the AAX SDK, v. 2.6.0. I open the .sln file to compile the SDK, and am presented with this dialog.
Unsure of which options to choose, any advice?
Options are, 10.0 latest, or no upgrade
and Toolset: upgrade to v143, or no upgrade
Basically, it is giving you the option to upgrade the project settings of the AAX SDK solution to your current version of Visual Studio. As the path suggests, the .sln file in the SDK was made with Visual Studio 2017.
…but all of the above shouldn’t be necessary any more if you are using a recent JUCE version, because last February a change has been made so that the AAX SDK is being compiled into JUCE directly and it does not need to link with AAXLibrary any more: