AIPMT 2010 Physics and Chemistry Paper

Ti3+(Z=22)
Ions which have unpaired electrons exhibit colour in solution. Ti3+ has an outer electronic configuration of 4s∘ 3d′, i.e., 1 unpaired electron. Thus its solution will be coloured.
Sc3*→d0
In case of La3+,4f0 configuration is present and in Lư, 4f14 is present. So, there is no possibility of f−f transition, hence these ions do not appear coloured.