D is correct!
Crispy, I think it's a matter of convenience, weight and cost... A crash cage is usually bigger, heavier, more expensive and just plain doesn't fit on a full fairing bike.... Well, there might be some that do, but they'd look silly, and nobody that I know of has made one for the 250...
Hence, in order to preserve the beautiful lines and body work of full fared motos, the industry has developed frame sliders, which are meant to do exactly the same thing, protect the frame and engine from impact, and has the added benefit of saving the plastics most of the time as well...
Or rather, I should say, frame sliders came first, and then with stunting growing in popularity, needing the added protection, the industry adapted to squeeze monies from them as well by designing pre-fabed, bolt on cages for naked stunt bikes...
If you've found something to the contrary, please share! I'm always up for some education!