We need to identify which species has a linear geometry.

Check each option using VSEPR / known molecular shape:

A: $O_3$ (ozone)

• Central O atom has two bond pairs and one lone pair.
• Electron pair geometry is trigonal planar, but molecular shape is bent.
• So, $O_3$ is not linear.

B: $Cl_2O$

• Structure is $Cl-O-Cl$ with oxygen as the central atom.
• Oxygen has two bond pairs and two lone pairs.
• Hence shape is bent, like $H_2O$.
• So, $Cl_2O$ is not linear.

C: $N_3^-$ (azide ion)

• Total valence electrons: $3\times 5 + 1 = 16$
• The ion is represented by resonance structures such as: $^-N=N^+=N^-$
• The central nitrogen has two regions of electron density and no lone pair causing bending.
• Hence the geometry is linear with bond angle $180^\circ$.
• So, $N_3^-$ is linear.

D: $NO_2$

• Nitrogen dioxide has an odd number of electrons.
• Around N, arrangement leads to a bent molecule.
• So, $NO_2$ is not linear.

Therefore, the only linear species is: $\boxed{N_3^-}$

Comparing with the stored correct answer: stored answer is C, which matches our result.