The aqueous solution containing which one of the following ion will be colourless ?

It is given to find out the colorless ion. 

Explanation

^(A) ${\mathrm{Sc}}^{3+}={\left[\mathrm{Ar}\right]}^{18}$- Because there are no unpaired electrons in the d-subshell, it is colorless.

(B) ${\mathrm{Fe}}^{2+}={\left[\mathrm{Ar}\right]}^{18}3{\mathrm{d}}^6$ - The presence of four unpaired electrons in the d-subshell gives it color.

(C)${\mathrm{Ti}}^{3+}={\left[\mathrm{Ar}\right]}^{18 }3{\mathrm{d}}^1$ - The presence of one unpaired electron in the d-subshell gives it color.
(D)${\mathrm{Mn}}^{2+}={\left[\mathrm{Ar}\right]}^{18}3{\mathrm{d}}^5$- The presence of 5 unpaired electrons in the d-subshell gives it color.

Hence the correct option is A) ${\mathrm{Sc}}^{3+}$.