(a) 21Sc3+=1s2,2s22p6,3s23p6 It is colourless due to absence of unpaired electrons in d-subshell. (b) 26Fe2+=1s2,2s22p6,3s23p63d6 It is colourless due to presence of four unpaired electrons in d-subshell. (c) 22Ti3+=1s2,2s22p6,3s23p63d1 It is coloured due to presence of one unpaired electron in d-subshell. (d) 25Mn2+=1s2,2s22p6,3s23p63d5 It is coloured due to 5 unpaired electrons in d-subshell.