I used Opera 9.26 today and so that under SafeSearchFilter:
Format Display Google web pages in:
(x) HTML (default)
( ) PDA
( ) XHTML

In Fx or IE I do not see that option

Just special for Opera?

I see this to, its most likely caused by a bug in Google's code, both the PDA and XHTML outputs are designed for devices with small screens thus Google is probably presuming your using Opera Mobile and providing the option.