It would be the one found on the Asus product page for that Mobo. Using yahoo search, 1st hit was this
https://www.asus.com/Motherboards/Z170-PRO-GAMING/
Click on Support
Click on Drivers & Tools
Select your OS in the drop down (I chose Win 10-64 bit)
Click on BIOS (4th one)
I see that the last BIOS posted was 2003 on 2016/09/29
I states that it "provides support for new CPUs".