Among the following which one will have the largest $\mathrm{O}-\mathrm{O}$ bond length?

${\mathrm{K}}_2{\mathrm{O}}_2$ will have the largest $0-0$bond length. 

The ${\mathrm{O}}_2^{-},{\mathrm{O}}_2,{\mathrm{O}}_2^{+}\text{and }{\mathrm{O}}_2^{2-}$  ions are present in${\mathrm{KO}}_2,{\mathrm{O}}_2,{\mathrm{O}}_2^{+}{\left[{\mathrm{AsF}}_6\right]}^{-}\text{, and }{\mathrm{K}}_2{\mathrm{O}}_2$  in the following bond orders: 1.5, 2, 2.5, and 1 respectively. $K_2{O}_2$ has the lowest bond order as a result. The $\mathrm{O}-\mathrm{O}$ bond length is therefore greatest in $K_2{O}_2$. Hence, option D is correct.