Which of the following cations in their respective oxidation states develops a colorless aqueous solution?

Why is the $E^{\Theta }$ value for the $Mn^{3+}/Mn^{2+}$ couple much more positive than that for $Cr^{3+}/Cr^{2+}$ or $Fe^{3+}/Fe^{2+}$? Explain.

Why do the transition elements exhibit variable oxidation states?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A):$ $Cu^{2+}$ in water is more stable than $Cu^{+}$.
Reason $(R):$ Enthalpy of hydration for $Cu^{2+}$ is much less than that of $Cu^{+}$.
In the light of the above statements,choose the correct answer from the options given below:

What is the correct order of atomic/ionic size?

Assertion : Borax bead test is not suitable for $Al(III)$.
Reason : $Al_2O_3$ is insoluble in water.