Why can't I see .Net 4.6 option for API compatibility layer?

I want to use the latest .Net version.

You can't with the official version of Unity.

It seems that the .Net version has only recently been updated to .Net 4.6 as described in this blog post:

This is a special version of Unity that is used for experimental purposes only. This is not intended to deliver your finished game.

.Net 4.6 is not officially supported. It worked and will probably be supported by the end of this year.

You can use the unofficial / experimental 5.6.0b5 version from this link . This will allow you to use .Net 4.6 in the editor. It won't work in assemblies because the runtime for other platforms is not yet complete.

If you download this you will help Unity improve its .Net 4.6. Don't download it because you want to release your game with APIs from .Net 4.6.

EDIT

For the latest experimental Windows, OSX, Linux standalone iOS with IL2CPP Android with IL2CPP and Mono are now supported.

There are still bugs to fix, though, and many platforms are not yet supported. This is why you shouldn't be releasing your game with this.

EDIT 2 :

Unity 2017.1.0b1 is now released and supports .NET 4.6. It is still in beta mode.

Go to menu "Edit" → "Project Settings" → "Player" → "Other Settings" → "Configuration" → "Script Version for Scripts" → .NET 4.6 Equivalent

then

Go to Edit → Project Settings → Player → Other Settings → Configuration → Api Compatibility Level → .NET 4.6