The correct order of bond enthalpy ($\mathrm{kJ~mol^{-1}}$) is :

C–C bonds bond energy = 348 kJ mol^-1 <br/><br/>Si–Si bonds bond energy = 297 kJ mol^-1 <br/><br/>Ge–Ge bonds bond energy = 260 kJ mol^-1 <br/><br/>Sn–Sn bonds bond energy = 240 kJ mol^-1 <br/><br/>$\therefore$ Correct order is - <br/><br/>$\mathrm{C-C > Si-Si > Ge-Ge > Sn-Sn}$ <br/><br/><b>Note :</b> <br/><br/>The property of forming bonds with atoms of the same element or tendency to self linking called catenation. <br/><br/>Carbon shows maximum catenation. On moving down the group catenation tendency decreases. This because the strength of $\mathrm{C}-\mathrm{C}$ bond is very high and in case of other elements, strength of ${M}-{M}$ (where ${M}={Si}, {G e}, {S n}, {P b}$ ) bond is decreases down the group.