Which one of the following is paramagnetic?

Assertion : $E^o$ for $Mn^{3+}/Mn^{2+}$ is more positive than $Cr^{3+}/Cr^{2+}$.
Reason : The third ionization energy of $Mn$ is larger than that of $Cr$.

Fill in the blanks:
$(a)$ The $4f$ and $5f$ series elements are known as lanthanoids and actinoids,respectively.
$(b)$ In transition elements,electrons are filled in the $d$-orbitals of the .......... energy levels.

The lanthanide contraction is responsible for the fact that

Consider $n$ as the number of lone pairs of electrons present in the equatorial position of the most stable structure of $ClF_3$. The ions from the following with $n$ number of unpaired electrons are :
$A. V^{3+}$
$B. Ti^{3+}$
$C. Cu^{2+}$
$D. Ni^{2+}$
$E. Ti^{2+}$
Choose the correct answer from the options given below :

Which of the following elements are included in the fourth transition series?