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

(a) 21Sc3+=1s2,2s2,2p6,3s2,3p6 It is colourless due to the absence of unpaired electrons in the d-subshell.
(b) 26Fe2+=1s2,2s2,2p6,3s2,3p63d6 It has colour due to the presence of four unpaired electrons in the d-subshell.
(c) 22Ti3+=1s2,2s2,2p6,3s2,3p63d1 It has colour due to the presence of one unpaired electron in the d-subshell.
(d) 25Mn2+=1s2,2s2,2p6,3s2,3p63d5 It has colour due to the presence of 5 unpaired electrons in the d-subshell.
Hence, option A is correct.