The use of blocks allows libraries of standard parts to be developed and saved for latter use. Blocks may be stored?

You can store them where you like , even in "the cloud"

(b) on disk, CDROM and the Internet.

I will go with option (b) on disk, CDROM and the Internet

B) is my answer and I also agree with Mr. Kinsbergen

Cad blocks can be stored both on disk and the internet storage sites.