Correct Statements Regarding Lanthanoid and Actinoid Elements

To determine the correct statements regarding lanthanoid and actinoid elements, we analyze each statement individually:

1. Statement A: $\text{Mo(VI)}$ and $\text{W(VI)}$ are less stable than $\text{Cr(VI)}$.

   • In Group 6 of the d-block elements ($\text{Cr}$, $\text{Mo}$, $\text{W}$), the stability of higher oxidation states increases down the group due to stronger metal-metal bonding and greater participation of $d$-orbitals.
   • Thus, $\text{Mo(VI)}$ and $\text{W(VI)}$ are more stable than $\text{Cr(VI)}$. For example, $\text{Cr(VI)}$ in $\text{Cr}_2\text{O}_7^{2-}$ is a strong oxidizing agent because it easily gets reduced, whereas $\text{MoO}_4^{2-}$ and $\text{WO}_4^{2-}$ are quite stable and not easily reduced.
   • Therefore, Statement A is incorrect.

2. Statement B: $\text{Ce}^{4+}$ and $\text{Tb}^{4+}$ are oxidant while $\text{Eu}^{2+}$ and $\text{Yb}^{2+}$ are reductant.

   • The most stable and common oxidation state for lanthanoids is $+3$.
   • $\text{Ce}^{4+}$ ($[\text{Xe}]4f^0$) and $\text{Tb}^{4+}$ ($[\text{Xe}]4f^7$) readily undergo reduction to achieve the stable $+3$ state ($\text{Ce}^{3+}$ and $\text{Tb}^{3+}$), so they act as strong oxidizing agents (oxidants).
   • $\text{Eu}^{2+}$ ($[\text{Xe}]4f^7$) and $\text{Yb}^{2+}$ ($[\text{Xe}]4f^{14}$) readily undergo oxidation to achieve the $+3$ state ($\text{Eu}^{3+}$ and $\text{Yb}^{3+}$), so they act as strong reducing agents (reductants).
   • Therefore, Statement B is correct.

3. Statement C: $\text{Cm}$ and $\text{Am}$ have seven unpaired electrons.

   • Americium ($\text{Am}$, $Z = 95$) has the electronic configuration $[\text{Rn}]\, 5f^7 7s^2$, giving 7 unpaired electrons.
   • Curium ($\text{Cm}$, $Z = 96$) has the electronic configuration $[\text{Rn}]\, 5f^7 6d^1 7s^2$, giving $7 + 1 = \mathbf{8\text{ unpaired electrons}}$.
   • Since Curium has 8 unpaired electrons, both elements do not have seven unpaired electrons.
   • Therefore, Statement C is incorrect.

4. Statement D: Actinoid contraction is greater from element to element than lanthanoid contraction.

   • The $5f$ electrons in actinoids have a poorer shielding effect than the $4f$ electrons in lanthanoids because $5f$ orbitals are more diffuse.
   • As a result, the increase in effective nuclear charge from element to element is greater in actinoids, leading to a larger contraction per element compared to lanthanoids.
   • Therefore, Statement D is correct.

Hence, the correct statements are B and D Only.

Correct Option: C ($\text{B and D Only}$)