I want to buy A12 9800 processor, will it be possible to install windows 7 on it
Windows 7 can be used with your processor since AMD has drivers for that OS: https://www.amd.com/en/support/apu/amd-series-processors/amd-a12-series-apu-for-desktops/7th-gen-a12-9800-apu .
But you need to go to your Asus Motherboard's Support to see how to install Windows 7 on your motherboard if it is 3xx or 4xx motherboard since USB is not supported when booting up. Thus you will need to use a PS/2 Keyboard and mouse and a Optical Driver to install Windows 7. Also need to download the Asus Chipset for Windows 7 or from AMD Support: https://www.amd.com/en/support
As you probably are aware, Windows 7 after January 20, 2020 won't be supported by Microsoft with Windows Updates. The OS will be considered to be Legacy. which probably means that after that date, AMD will probably stop updating its Windows 7 drivers and so will other manufacturers of their hardware eventually.
I would highly suggest to try in Ingrade Upgrade to Windows 10. It is possible you may still get it activated for free instead of purchasing a Windows 10 license. If it doesn't activate for free then you need to purchase a Windows 10 license or revert back to Windows 7 without any future support.
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