Windows 8.1 came pre-installed with Dell Inspiron 15 3531 if you purchase it. And for Windows 7 fans, you might want to install this OS in this laptop.
This laptop has the drivers for Win 7 OS, so you can do the downgrade.
Before you do this, you should have backup your laptop, if in case you want to install Windows 8.1 OS back. This means, everything in your laptop will be gone or all wiped after Windows 7 installation process, you won’t see any of your previous files on your computer.
Since this Inspiron 15 3531 model doesn’t come with a Pro version of Windows 8.1, you will need to buy Windows 7 license key. If it is a retail disc, you will need Win 7 Service Pack 1.