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: