The reaction for obtaining the metal (M) from its oxide (M₂O₃) ore is given by

$${M_2}{O_3}(s) + 2Al(l)\buildrel {Heat} \over \longrightarrow A{l_2}{O_3}(l) + 2M(s)$$, (s = solid, l = liquid) in that case, M is

At room temperature, the reaction between water and fluorine produces

$$[X] + Dil.\,{H_2}S{O_4} \to [Y]:$$ Colourless, suffocating gas

$$[Y] + {K_2}C{r_2}{O_7} + {H_2}S{O_4} \to $$ Green colouration of solution.

Then, [X] and [Y] are

The correct basicity order of the following lanthanide ions is