Jellybean(might be 1 revision short of the end of the Jellybean version).
https://www.samsung.com/us/support/SupportOwnersFAQPopup.do?faq_id=FAQ00047453&fm_seq=51497
Samsung supported for 2 years and then drops support.
Unless you load a custom Android(open source community), then you could get KitKat. Lots of Lots of reading before you try flashing a new version of Android.
http://www.cyanogenmod.org/
http://forum.cyanogenmod.com/
http://forum.cyanogenmod.com/topic/74524-can-i-get-cyanogenmod-on-my-device/
