"Safari opens with:" option missing
I have 2 Macs, both running the latest OS X update with Safari 9.1 (11601.5.17.1). On one machine, the first option under Preferences>General is "Safari opens with:" but this line is missing in the general preferences on the other machine. Why the difference? Can I add this option to the version where it is missing?
Mac mini (Late 2012), OS X El Capitan (10.11)