Tp start with here is the Toshiba Satellite C850 Users Guide: User Guide.
Here's the spec sheet: Toshiba Satellite C850
Your system uses: PC3-10600/12800 DDR3-1333 SODIMM 204pin memory. As to the memories speed You can use a faster rated module (1600MHz) but you shouldn't use a slower than what is spec'ed. In your case the speed needed is 1333 MHz.
Here's a little more detail on DDR3 memory: DDR3 SDRAM
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
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