USNCO Question - Determining solute using spectrophotometer

Below is a question from the 2017 USNCO:


The concentration of which approximately 0.01M solution could be most accurately determined by a visible spectrophotometer (or colorimeter)?


(A) $\ce{Mn(NO3)2}$


(B) $\ce{Co(NO3)2}$


(C) $\ce{Zn(NO3)2}$


(D) $\ce{Pb(NO3)2}$


The answer sheet states B is the correct answer.


I researched the colours of the ions in solution, and found:


* Mn is pink
* Co is pink
* Zn is clear
* Pb is clear


I was unsure how to progress from there.


How should I approach this question in an exam if there was no reference book?




An aqueous solution of $\ce{Mn^2+}$ is only faintly pink, whereas that of $\ce{Co^2+}$ is much more colored: 


[![some ionic solutions; image source http://staff.buffalostate.edu/nazareay/che112/3d-22.JPG](https://i.stack.imgur.com/PDBLM.jpg)](https://i.stack.imgur.com/PDBLM.jpg)


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