Which of the following elements is found in the human body?

In the following pairs,the one in which both transition metal ions are colourless is

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 $Cr$.

Which of the following does not show more than one oxidation state?

For what reason is the element $Th$ included in the $f$-block?

What are $f$-block elements? Give their characteristics.