$3d-$ metal ions exhibit paramagnetic behavior because . . . . .

The correct statement$(s)$ about $Cr^{2+}$ and $Mn^{3+}$ is(are)
[Atomic numbers of $Cr = 24$ and $Mn = 25$]
$(A)$ $Cr^{2+}$ is a reducing agent
$(B)$ $Mn^{3+}$ is an oxidizing agent
$(C)$ Both $Cr^{2+}$ and $Mn^{3+}$ exhibit $d^4$ electronic configuration
$(D)$ When $Cr^{2+}$ is used as a reducing agent,the chromium ion attains $d^3$ electronic configuration

Higher second ionisation enthalpies than expected are observed for

Which of the following elements form interstitial compounds?

$A$ metal $M$ has the electronic configuration $M: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$. To which block does this metal belong?

Four successive members of the first series of the transition metals are listed below. For which one of them is the standard electrode potential $(E^o_{M^{2+}/M})$ value positive?