Which of the following elements has a completely filled $4f$ orbital in its ground state electronic configuration?

What are the different oxidation states exhibited by the lanthanoids?

Most common oxidation states of $Ce$ $(cerium)$ are

Consider the following statements:
$I$. $La(OH)_3$ is the least basic among hydroxides of lanthanides.
$II$. $Zr^{4+}$ and $Hf^{4+}$ possess almost the same ionic radii.
$III$. $Ce^{4+}$ can act as an oxidizing agent.
Which of the above is/are true?

An element with electronic configuration $[Rn]\, 7s^2\, 5f^3\, 6d^1$ will fall in

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 .......... .