The transition metal used in $X$-ray tubes is:

Which element exhibits a $+7$ oxidation state?

The higher oxidation states of transition elements are found to be in combination with $A$ and $B,$ which are

Assertion : Transition metals show variable valency.
Reason : Transition metals have a large energy difference between the $ns^2$ and $(n-1)d$ electrons.

Match List-$I$ with List-$II$

List-$I$ Species	List-$II$ Electronic distribution
$A$. $Cr^{2+}$	$I$. $3d^8$
$B$. $Mn^{+}$	$II$. $3d^3 4s^1$
$C$. $Ni^{2+}$	$III$. $3d^4$
$D$. $V^{+}$	$IV$. $3d^5 4s^1$

Choose the correct answer from the options given below:

Identify the correct decreasing order of atomic size from the following.