Of the bonds c-c, cc, and c≡c, the c-c bond is

 The line in between indicates that there is a single bond connecting the two carbon atoms. Atoms can be bonded via single bond, double bond and a triple bond. Among the three, the single bond is the weakest, but it allows for more atoms to connect to it because it leaves 3 more pairs available for bonding.

So in summary, among all the bonds that was given, a c-c bond is the weakest and the longest.