Which of the following hydrocarbon has least C−C bond length ?

Let us see the bond order in each molecule

In ${\mathrm{C}}_2{\mathrm{H}}_2,\mathrm{C}\equiv \mathrm{C} \mathrm{bond} \mathrm{order} \mathrm{is} 3$

In ${\mathrm{C}}_6{\mathrm{H}}_6,\mathrm{C}=\mathrm{C} \mathrm{bond} \mathrm{order} \mathrm{is} 2$

In ${\mathrm{C}}_2{\mathrm{H}}_4,\mathrm{C}=\mathrm{C} \mathrm{bond} \mathrm{oder} \mathrm{is} 2$

In ${\mathrm{C}}_2{\mathrm{H}}_6,\mathrm{C}-\mathrm{C} \mathrm{bond} \mathrm{order} \mathrm{is} 1$

Bond length and bond order are inversely proportional to each other. For a molecule if bond order is high then bond length to be short. Among the given all molecules, acetylene ${\mathrm{C}}_2{\mathrm{H}}_2$ has least C-C bond length. Since the bond order is high in acetylene than the remaining all molecules.