In the mercury cathode cell, sodium and not hydrogen is discharged at the cathode mainly because

Ahydrogen has a very high overpotential on mercury, and sodium dissolves in mercury as an amalgam
Bthe standard reduction potential of $Na X^{+}$ is higher than that of water
C $Na X^{+}$ is present in higher concentration than $H X^{+}$ only at the anode
Dmercury reacts with $H X^{+}$ to form $HgH X_{2}$

Answer: A

The standard reduction potential of

Na X^{+}

(

- 2.71 V

) is far lower than that for water reduction, so thermodynamics alone favours

H X_{2}

. The high overpotential for

H X_{2}

evolution on mercury, together with the energy gained by forming the amalgam, makes sodium discharge instead.