Which oxidation state is common for lanthanoid elements?

The elements in which electrons are progressively filled in $4f$ orbital are called

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 $(B)$.

Which of the following statements concerning lanthanide elements is false?

Which of the following statements is wrong regarding lanthanoids?

$4f^2$ electronic configuration is found in