Q.State as to why

(a) A solution of Na$_2$CO$_3$ is alkaline

Na$_2$CO$_3$ is the salt of a strong base (NaOH) and a weak acid (carbonic acid, H$_2$CO$_3$). In water, the carbonate ion undergoes hydrolysis:

This hydrolysis releases hydroxide ions, making the solution basic (alkaline).

(b) Alkali metals are prepared by electrolysis of their fused chlorides

Alkali metals are extremely reactive and immediately react with water, so if an aqueous solution of the chloride were electrolysed, the metal formed at the cathode would react instantly with the surrounding water; furthermore, water (H$_2$O/H$^+$) is preferentially reduced at the cathode over the alkali metal ion, so H$_2$ gas, not the metal, would be liberated. Using the fused (molten, water-free) chloride avoids both problems, allowing the metal ion to be reduced cleanly to the free metal at the cathode.

(c) Sodium is found to be more useful than potassium …

Treat each part independently: (a) apply salt-hydrolysis rules to a strong-base/weak-acid salt;

(b) reason from the relative ease of reduction of water versus the alkali metal ion, plus the metal's reactivity with water, to ju …

• For (a), calling Na$_2$CO$_3$ a 'strong base' itself rather than correctly explaining that its alkalinity arises from hydrolysis of the carbonate ion in solution.
• For (b), giving only 'because they are reactive' without explaining the two specific reasons (preferential reduction of water at the cathode, and reaction of the liberat …