“Do you want to specify an alternate source path? One or more installation selections are missing source files…”
To solve this, you can either:
1. Go to a command prompt and enter this:
dism /online /enable-feature /featurename:NetFX3 /all /Source:d:\sources\sxs /LimitAccess
Note: Source should be the Windows installation disc. In my case, this was located on D:
2. Go down to “Specify an alternate source path” and enter “d:\sources\sxs” as the path.
Note, if you need parent feature to be enabled first, use the /all option to automatically enable each parent feature.
In the Confirm installation selections window, before click Install, click the Specify an alternate source path and provide the path as D:\Sources\sxs (where D is the drive letter where the Windows Server 2012 R2 media is mounted).