Correct answer is: A double bond between carbon atoms is longer than a triple bond between carbon atoms.
Reason:
In a carbon-carbon double bonded system, each carbon has undergoes sp2 hybridization. Thus, it bond order of system is 2. While in case of carbon-carbon bonded system via triple bond, each carbon atom undergoes sp hybridization. The bond order in this case is 3. Further, extent of overlap in sp hybridized orbitals is more as compared to sp2 hybridized orbitals. Hence, a double bond between carbon atoms is longer than a triple bond between carbon atoms.