Which element from the following possesses half-filled $d$-orbitals either in expected or in observed electronic configuration?

The size of $Be^{2+}$ and $Al^{3+}$ is not so close,but they show a diagonal relationship due to:

With the help of the following reactions,arrange the metals $A$,$B$,$D$,and $E$ in decreasing order of their reactivity:
$(I)$ $B + ANO_3 \longrightarrow BNO_3 + A$
$(II)$ $A + HCl \longrightarrow ACl + \frac{1}{2} H_2$
$(III)$ $D + ECl \longrightarrow DCl + E$
$(IV)$ $D + HNO_3 \longrightarrow$ $H_2$ gas is not evolved

Identify the group and valency of the element having atomic number $119$. Also,predict the outermost electronic configuration and write the general formula of its oxide.

Identify the set of acidic oxides.

Which one of the following orders correctly represents the increasing acid strengths of the given acids?