Which of the following pairs of elements does $NOT$ represent chemical twins?

For $Cr, Mn, Fe,$ and $Co$,the $E_{{M^{3+}}/{M^{2+}}}^{\circ}$ values are $0.41, +1.57, +0.77,$ and $+1.97 \ V$ respectively. Which of these metals is most easily oxidized from the $+2$ to the $+3$ oxidation state?

Which of the following ions will be colored in an aqueous solution?

$A$ transition metal $X$ has a configuration $[Ar]3d^4$ in its $+3$ oxidation state. Its atomic number is

Which of the following elements is man-made (synthetic)?

What is the total number of transition series of $d$-block elements?