Hard reset on W10M, Lumia 640

[19930105]

When I hard reset my phone, which OS will be back? The 8.1 or the W10?

 

[div3rsion]

I'll get the version back that was on it when you hard reset it. So if you where on W10M already, you stay there.
If you want to go back to W8.1 you have to use the Windows Device Recovery Tool.