Which of the following elements does not exhibit variable valency?

Which of the following statements is not true?

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

Niobium $(Nb)$ and ruthenium $(Ru)$ have " $x$ " and " $y$ " number of electrons in their respective $4d$ orbitals. The value of $x+y$ is $...........$

$A$ first row transition metal in its $+2$ oxidation state has a spin-only magnetic moment value of $3.86 \ BM$. The atomic number of the metal is

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: The first ionization enthalpy of $3d$ series elements is more than that of group $2$ metals.
Reason $(R)$: In $3d$ series of elements,successive filling of $d$-orbitals takes place.
In the light of the above statements,choose the correct answer from the options given below: