Which of the following oxidation states is the most common among the lanthanoids?

Most basic hydroxide among the following is

Which series of elements are radioactive?

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ All actinoid elements have a $7s^1$ configuration.
$(b)$ All actinoid elements have a $7s^0$ configuration.
$(c)$ There are no electrons in the $7s$ orbital of actinoids.

Fill in the blanks:
$1.$ The size of $M^{3+}$ ions in the actinoid series ........... with an increase in atomic number.
$2.$ The common oxidation state of elements in the actinoid series is .......... .
$3.$ The appearance of elements in the actinoid series is like .......... .

Among $Ce(4f^{1} 5d^{1} 6s^{2})$,$Nd(4f^{3} 6s^{2})$,$Eu(4f^{7} 6s^{2})$ and $Dy(4f^{10} 6s^{2})$,the elements having highest and lowest $3^{rd}$ ionisation energies,respectively are