Which of the following is correct decreasing order of bond length regarding $\mathrm{N}_2, \mathrm{O}_2$ and $\mathrm{Cl}_2$ ?

Bond lengths decrease with increasing bond multiplicity and increase with larger atomic radii. The bond order and atomic size determine the bond length: $\mathrm{N}_2$ has a triple bond, $\mathrm{O}_2$ a double bond, and $\mathrm{Cl}_2$ a single bond.

$\mathrm{N}_2$: $\mathrm{N} \equiv \mathrm{N}$, bond length $\approx$ 109.8 pm
$\mathrm{O}_2$: $\mathrm{O} = \mathrm{O}$, bond length $\approx$ 121 pm
$\mathrm{Cl}_2$: $\mathrm{Cl} - \mathrm{Cl}$, bond length $\approx$ 199 pm

Since higher bond order corresponds to shorter bonds, and chlorine has the largest atomic radius, the order is $\mathrm{Cl}_2 > \mathrm{O}_2 > \mathrm{N}_2$.