$A$ reduction in atomic size with an increase in atomic number is a characteristic of elements of

Lanthanoid ions with $4f^7$ configuration are :
$A$. $Eu^{2+}$
$B$. $Gd^{3+}$
$C$. $Eu^{3+}$
$D$. $Tb^{3+}$
$E$. $Sm^{2+}$
Choose the correct answer from the options given below :

The electronic configuration of Curium (Atomic number $96$) is

The lanthanide contraction relates to

Identify the correct statement from the following:

Identify the formula of compound $(B)$ formed in the following reaction:
$(i)$ Lanthanoids $(Ln)$ are burnt in oxygen to form compound $(A)$.
$(ii)$ Compound $(A)$ reacts with $CO_2$ in excess to form compound $(B)$.