Why are alkali metals highly reactive?

Which of the following statements is correct?

The chloride of an element $A$ gives a neutral solution in water. In the periodic table,the element $A$ belongs to

The stability of the following alkali metal chlorides follows the order:

Magnesium is manufactured by electrolysing fused magnesium chloride using

$A$ metal $(A)$ on heating in nitrogen gas gives compound $(B)$. $(B)$ on treatment with $H_{2}O$ gives a colourless gas which when passed through $CuSO_{4}$ solution gives a dark blue-violet coloured solution. $(A)$ and $(B)$ respectively,are