Explain the meaning of transition series.

Out of the following identify the d-block element.

$\mathrm{Ca}, \mathrm{Mn}, \mathrm{U}, \mathrm{Al}$

Account for the given statement: The outer electronic configuration of ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ is ${\mathrm{d}}^4$ but ${\mathrm{Cr}}^{2+}$ is reducing agent and ${\mathrm{Mn}}^{3+}$ is oxidising.