Which element from the following exhibits various different oxidation states from $+2$ to $+7$?

$A$ transition element $X$ has a configuration $[Ar]3d^4$ in its $+3$ oxidation state. Its atomic number is

Cupric compounds are more stable than their cuprous counterparts in the solid state. This is because

Reactivity of transition elements decreases almost regularly from $Sc$ to $Cu$. Explain.

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:

Which element from the following exhibits the highest number of various different possible oxidation states?