Which of the following is ferromagnetic in nature?

Discuss the magnetic properties of transition elements.

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:

Most stable cation in aqueous medium is

Why do $CrO_3$ and $V_2O_5$ have low melting points?

Higher second ionisation enthalpies than expected are observed for