Which transition element does not exhibit variable oxidation states?

In the electronic configuration of which of the given elements,$Aufbau$ principle or $(n+l)$ rule is violated?

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:

Highest density is of $...$.

Which of the following ions indicates the maximum magnetic moment?

Which of the following statements is not correct? $(Ve = 58, \, Lu = 71, \, La = 57, \, Yb = 70)$