Install disk restart holding 'C' or 'Option' ?

DU tells me 'the volume macintoshHD needs to be repaired'
error: the underlying task reported failure on exit
volume needs repair

so i need to startup from my install disc
ok i've had to do that before

but some resources say to restart while holding down the 'C' key, others the 'Option' key.

i think i've done it before using the 'option' key. not sure now.

my question:
-what's the definitive one to use for this?
-and what's the difference? (i've read what they do differently but don't really understand the difference)

Also:
my AppleCare just ran out and i understand it can't be renewed. is there another option available? or is apple's 'pay per incident' the only way from now on?

thank you all!
this is an invaluable forum

 

so it doesn't really matter then
the 'option' just adds another step in this instance
thank you