Froth flotation method is useful for the concentration of which of the following ores?

In the extraction of aluminium:
Process $X$: applied for red bauxite to remove iron oxide (chief impurity).
Process $Y$: (Serpeck's process) applied for white bauxite to remove $Z$ (chief impurity).
Then,process $X$ and impurity $Z$ are:

Which one of the following is true?

$NaCN$ is sometimes added in the froth flotation process as a depressant when $ZnS$ and $PbS$ minerals are expected because

Which substance is used as a froth stabilizer in the froth flotation process?

Which one of the following is not a method of concentration of metals?