Which general symbol is used to represent lanthanoids?

The outer electronic structure of lawrencium (atomic number $103$) is

In lanthanides,with increase in atomic number,atomic radius decreases,except for the element $\underline{ X }$. What is $\underline{ X }$?

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.

Europium is

Although $(+3)$ oxidation state is the characteristic oxidation state of lanthanoids,cerium shows $(+4)$ oxidation state also. Why?