Which of the following compounds is used in gas lighters?

The ion with $4f^7$ configuration is

Select the incorrect statement about lanthanides.

The correct order of ionic radii of $Ce$,$La$,$Pm$ and $Yb$ in $+3$ oxidation state is

$Ln$ (Lanthanide) $\xrightarrow{\text{With Halogen}} X$
$Ln$ (Lanthanide) $\xrightarrow{\text{Burn With } O_2} Y$
$Ln$ (Lanthanide) $\xrightarrow{\text{Heated with } N_2} Z$
$X, Y$ and $Z$ are respectively:-

Identify the isoelectronic pair of ions from the following.