Use Software Manager in Yast2. Should keep default or desktop. Depending on which version of openSUSE you have and what the installer detects determines which kernel you get. My 11.3 desktop has default, my 11.4_64 server has desktop. If you did not install the other kernels, then they were install has needed software. Looks like you may have choosing kernel development patterns in Software Management. When you boot you should see which kernel is loaded at startup, keep that one.
OK i resolved that.
but i recently update the kernel with yast online update and after that i recognized that there is two option in the boot, one is opensuse default and another is opensuse desktop, and both versions are the same (before updating the kernel there was only one option in the boot except fail safe).
which one should i select and which one can i remove?
Please do not remove the same flavor that you use while you do the removing/deinstalling.
Just keep the version that you like better. As I understand it, the desktop flavor is more optimized in view of performance (including faster booting) and the default flavor is more optimized in view of low power consumption (there was a test on page 22-30 in the latest German 3/12 “Linux Magazin” - probably something alike would be available in a English language ‘sister’). And the pae flavor (pae means physical address extension) will allow you to use more than 4 GiByte RAM without real 64bit software or hardware.
Compare: Kernel - openSUSE Physical Address Extension - Wikipedia