This is the correct one for a C5, that fits w/o modification:
OPTIMA® Batteries 8022-091 75/25 REDTOP® Starting Battery :: OPTIMA® Batteries
The 75/25 is the group size, and is all you need to remember if you're headed to a store to buy one.
They also make a larger 34/78 size, which will physically fit in the battery compartment, but it requires a different clamp (avail from Optima) to secure it.
I used to have a red top, but after it failed, and receiving poor warranty service about it, I quit buying Optima. IMO, their quality isn't what it used to be, and they aren't worth the premium price.