The first element of the rare-earth metals is:

Most basic hydroxide among the following is

$4f^2$ electronic configuration is found in

Explain the lanthanoid contraction.

Determine whether the following statements are True $(T)$ or False $(F)$:
$(a)$ Actinoid contraction is greater than lanthanoid contraction.
$(b)$ Actinoids are elements of the $4f$ series.
$(c)$ All actinoid elements have a $7s^2$ configuration.

In the lanthanide series,the element well known to exhibit $+4$ oxidation state is