A 1:3 electrolyte in an aqueous solution is

A 1:3 electrolyte dissociates in an aqueous solution to give one cation and three anions, or vice versa, producing a total of four ions per formula unit.

$[\text{Co(NH}_3)_3(\text{NO}_2)_3]$ is a neutral complex and does not dissociate into ions. It is a non-electrolyte.

$[\text{CoCl}_2(\text{NH}_3)_4]\text{Cl} \rightarrow [\text{CoCl}_2(\text{NH}_3)_4]^{+} + \text{Cl}^{-}$ (1:1 electrolyte)

$[\text{CoCl(NH}_3)_5]\text{Cl}_2 \rightarrow [\text{CoCl(NH}_3)_5]^{2+} + 2\text{Cl}^{-}$ (1:2 electrolyte)

$[\text{Co(NH}_3)_6]\text{Cl}_3 \rightarrow [\text{Co(NH}_3)_6]^{3+} + 3\text{Cl}^{-}$ (1:3 electrolyte)

Thus, $[\text{Co(NH}_3)_6]\text{Cl}_3$ is a 1:3 electrolyte.

Answer: $[\text{Co(NH}_3)_6]\text{Cl}_3$