(a) On the basis of incompletely filled 3d-orbitals in case of Scandium atom (3d1) having one electron in its ground state, it is regarded as a transition element. On the other hand, zinc atom having completely filled d-orbitals (3d10) in its ground state as well as in its oxidized state, it is not considered as a transition element.
(b) [Ti(H2O)6]+3 is coloured because Ti+3(3d1) has one unpaired electron which undergoes d−d transition so it is coloured. While [Sc(H2O)6]+3 (3d0) has no unpaired electrons so it will not undergo d−d transition so it is colourless.
(c) Physical and chemical properties of 4d and 5d series elements are similar because they have same atomic and ionic radii due to the lanthanide contraction. Due to equal atomic radii the size of Zr and Hf,Nb and Ta,Mo and W, etc., the two elements of each pair have the same properties.