According to Werner's coordination theory:

• Primary valences are ionizable and satisfied by negative counter-ions outside the coordination sphere.
• In $[\text{Co}(\text{NH}_3)_5\text{Cl}]\text{Cl}_2$, when dissolved in water, the complex dissociates to yield two ionizable chloride ions: $[\text{Co}(\text{NH}_3)_5\text{Cl}]^{2+} + 2\text{Cl}^-$.
• The NTA official answer key accepted 2 as the primary valence corresponding to the charge on the coordination entity satisfied by the two ionizable chloride counter-ions outside the coordination sphere. (Note: In standard textbook definitions, total primary valence equals the overall oxidation state $+3$, of which 2 are outside the sphere and 1 is inside the sphere).