(1)

Ferromagnetism is an extreme form of paramagnetism hence, statement-I is true.

Electronic configuration of ${\mathrm{Cr}}^{2+}:\left[\mathrm{Ar}\right]3{\mathrm{d}}^4$ thus, number of unpaired electrons is 4. Electronic configuration of ${\mathrm{Nd}}^{3+}:\left[\mathrm{Xe}\right]4{\mathrm{f}}^3$ thus, number of unpaired electrons is 3. Hence, statement-II is false.

(1)

The formation of Ce(IV) is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The $\mathrm{E}°$ value for ${\mathrm{Ce}}^{4+}/{\mathrm{Ce}}^{3+}$ is +1.74 V which suggests that it can oxidise water. Tb(IV) has half-filled $f$-orbitals and is an oxidant.

(2)

Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions. Colour of these ions may be attributed to the presence of $f$-electrons. Neither ${\mathrm{La}}^{3+}$ nor ${\mathrm{Lu}}^{3+}$ ion shows any colour but the rest do so. However, absorption bands are narrow, probably because of the excitation within $f$-level (orbitals).

(1)

Among the given lanthanoid ions, $C{e}^{4+}$ and $Y{b}^{2+}$ are diamagnetic in nature. 

Electronic configuration of $Ce:\left[Xe\right]4f^{1}5d^{1}6s^2$ and $C{e}^{4+}:\left[Xe\right]$

Electronic configuration of $Yb:\left[Xe\right]4f^{14}6s^2$ and $Y{b}^{2+}:\left[Xe\right]4f^{14}$

(2)

Due to high exchange enthalpy, $G{d}^{3+}\left(4f^7\right)$ acquires extra stability and has low third ionisation enthalpy.

(3)

The first few members of the lanthanoid series are quite reactive, almost like calcium. However, with increasing atomic number, their behaviour becomes similar to that of aluminium.

(2)

(2)

(1)

Zr and Hf have nearly same radii due to lanthanoid contraction.

(1)

Due to poor shielding effect of $4f$-orbitals, nucleus will exert a strong attraction and size of atom or ion goes on decreasing as move in the series with increase in atomic number.