The correct statements among the following are,
A. $Mo(VI)$ and $W(VI)$ are less stable than $Cr(VI)$.
B. $Ce^{4+}$ and $Tb^{4+}$ are oxidant while $Eu^{2+}$ and $Yb^{2+}$ are reductant.
C. $Cm$ and $Am$ have seven unpaired electrons.
D. Actinoid contraction is greater from element to element than lanthanoid contraction.

Choose the correct answer from the options given below:

In d-block elements, the stability of higher oxidation states increases down a group. Therefore, $Mo(VI)$ and $W(VI)$ are more stable than $Cr(VI)$. Statement A is incorrect.

The most stable oxidation state for lanthanoids is $+3$. Ions like $Ce^{4+}$ and $Tb^{4+}$ tend to gain an electron to achieve the $+3$ state, acting as oxidants. Conversely, $Eu^{2+}$ and $Yb^{2+}$ tend to lose an electron to achieve the $+3$ state, acting as reductants. Statement B is correct.

The electronic configuration of $Am$ is $[Rn] 5f^{7} 7s^{2}$, which has $7$ unpaired electrons. The electronic configuration of $Cm$ is $[Rn] 5f^{7} 6d^{1} 7s^{2}$, which has $8$ unpaired electrons ($7$ in $5f$ and $1$ in $6d$). Statement C is incorrect.

The $5f$ electrons have a poorer shielding effect compared to $4f$ electrons. As a result, the effective nuclear charge increases more significantly across the actinoid series, leading to a greater actinoid contraction from element to element than the lanthanoid contraction. Statement D is correct.

Thus, only statements B and D are correct.

Answer: B and D Only