'C−C' bond length in benzene lies between single and double bond.The reason is
Resonance
Some of the characteristics of resonance are:
(i) It gives identical bonding (concept of partial double bond in resonance hybrid) and hence identical bond length. For example, in benzene, all 'C−C' bonds are of 140 pm long.
(ii) It leads to the shortening of single 'C−C' or other bonds in resonance hybrid.
Resonance Hybrid of Benzene is
Partial double bond on each carbon