Transition metals have variable oxidation states, form complex ions with ligands (ligands donate electron pairs into the metal centre, so the transition metal is behaving as a Lewis acid), have coloured compounds, and display catalytic and magnetic properties. Thus 1,2 and 3 is the correct answer.

A ligand must have a lone pair of electrons that can be donated to a metal centre to form a coordinate (dative) bond. All of the species written have a lone pair except for the ammonium ion (NH₄⁺) which does not, and is therefore the correct answer.

The 'en' ligand is 1,2-ethanediamine and its structure is shown in the data book (section 16 Ligands). As can be seen from the structure it has two amine groups capable of donating a pair of electrons and it is therefore bidentate.

If each 'en' forms two coordinate bonds to the nickel ion, and there are three ligands, there are a total of six coordinate bonds into the nickel centre - the coordination number is 6.

'6 ; bidentate' is the correct answer.

Most transition metal compounds are coloured, but Zn is not considered to be a transition metal (despite being in the d-block) because it does not form ions with incomplete d-orbitals (It is only stable as Zn²⁺ in which it has lost both 4s electrons and has a complete 3d sub-level). Without incomplete d-orbitals in the metal d-block compounds cannot be coloured, so Zinc compounds, like ZnO, are not coloured. ZnO is the correct answer.

ZnO is a white solid, CuCO₃ is a blue-green solid, Fe₂O₃ is a red-brown solid, [Fe(H₂O)₅SCN]²⁺ is a blood-red solution.

The charge on the copper ion is 2+ (given by the Roman numerals, II). Each chloride ligand (four are found in the complex) has a charge of 1−, so the overall charge will be 2+ + (4×1−) = 2−.

Charge is always expressed as number then sign, that is 2−.

Ferromagnetic substances, like iron, are permanently magnetic.

Paramagnetic substances have unpaired electrons, and behave as if they are weakly magnetic when an external magnetic field is applied.

Diamagnetic substances have all electrons paired, are not magnetic at all, and may be weakly repelled by an external magnetic field.

Thus paramagnetism is the correct answer here.

Transition metals can often act as catalysts because of their ability to switch between oxidation states, and because substances can easily adsorb onto the surface of a metal.

There is no evidence that the relatively high melting points of transition metals contributes to their catalytic activity.

Thus '2 and 3 only' is the correct answer.

Transition metals do not have low first ionisation energies when compared to the main group metals.

The electrons in singly occupied d-orbitals are relatively unstable, and the s and d sub-levels are very close to each other in energy (remember that 's-electrons' are gained first and lost first). These factors contribute to transtion metals having successive ionisation energies that do not have 'big jumps' and thus being able to have variable oxidation states; thus 2 and 3 only is the correct answer.

Paramagnetic substances have unpaired electrons, and behave as if they are weakly magnetic when an external magnetic field is applied.

Diamagnetic substances have all electrons paired, are not magnetic at all, and may be weakly repelled by an external magnetic field.

Oxygen and vanadium atoms both have unpaired electrons in their electronic configurations (1s² 2s² 2p⁴ and 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d³ respectively), but zinc atoms do not (1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰). Remember that all orbitals in a given sub-level are filled singly first, before electrons pair-up.

Thus '1 and 3 only' is the correct answer.