Which element can have oxidation state from $4$ to $6$?

Which of the following compounds reacts with $NaOH$ and $Na_2O_2$ to produce a yellow color?

The order of basic character for the monoxides of transition metals is . . . . .

How many statements are incorrect?
$(a)$ $Mn^{+3}$ and $Co^{+3}$ are strong oxidising agents.
$(b)$ $Ti^{+2}$,$V^{+2}$ and $Cr^{+2}$ are strong reducing agents.
$(c)$ $CrO_3$ is acidic in nature.
$(d)$ $E^{\circ}_{M^{+3}/M^{+2}} = -ve$ for $Fe$.

Passivity of iron is due to the formation of

The highest oxidation state of $Mn$ is shown by