'X' melts at low temperature and is a bad conductor of electricity in both liquid and solid state. X is:

We begin by recalling some fundamental bonding concepts:

1. Metals possess a sea of delocalised electrons, so they conduct electricity in the $$\text{solid}$$ as well as in the $$\text{liquid}$$ state. Their melting points are usually moderate to high.

2. Ionic compounds (for example, sodium chloride) consist of oppositely charged ions held together by strong electrostatic forces. Therefore, they generally have very high melting points. They are $$\text{non-conductors}$$ in the solid state because the ions are locked in position, but they become $$\text{good conductors}$$ when molten or dissolved in water, as the ions are then free to move.

3. Giant covalent (network) solids such as $$\text{SiC}$$ or diamond have extremely strong covalent bonds throughout the lattice. Consequently, they melt only at very high temperatures and are usually $$\text{electrical insulators}$$ (diamond) or at best semiconductors (SiC).

4. Simple (discrete) covalent molecular substances are held together internally by covalent bonds but interact with one another only through weak van der Waals forces. Because these intermolecular forces are weak, the melting points are low. In addition, there are no free ions or mobile electrons, so these substances are $$\text{poor conductors}$$ of electricity in both the solid and the liquid state.

Now we examine each option in the light of these points:

(A) Zinc sulphide, $$\text{ZnS}$$

Zinc sulphide crystallises in a close-packed lattice akin to $$\text{ZnS}$$ (zinc-blend) structure. The bonding is largely ionic with a degree of covalency. Because of the strong lattice, its melting point is very high (well above $$1000\ ^\circ\text{C}$$). Although it is an electrical insulator in the solid state, its molten form contains mobile $$\text{Zn}^{2+}$$ and $$\text{S}^{2-}$$ ions and would thus conduct electricity. Therefore, it does not match the given description.

(B) Mercury, $$\text{Hg}$$

Mercury is a metal. A characteristic property of metals is the presence of delocalised electrons; hence mercury conducts electricity in both states. Although mercury indeed melts at the unusually low temperature of $$-38.9\ ^\circ\text{C}$$, the key phrase "bad conductor of electricity in both liquid and solid state" rules it out.

(C) Silicon carbide, $$\text{SiC}$$

Silicon carbide is a giant covalent (network) crystal similar to diamond. Its melting point is extremely high (about $$2730\ ^\circ\text{C}$$), immediately disqualifying it, since the substance in question melts "at low temperature." Moreover, while pure $$\text{SiC}$$ is an insulator at room temperature, at elevated temperatures it shows semiconducting behaviour, so electrical properties still do not fully satisfy the requirement.

(D) Carbon tetrachloride, $$\text{CCl}_{4}$$

Carbon tetrachloride is a simple covalent molecule. The only attractive forces between individual $$\text{CCl}_{4}$$ molecules are weak London dispersion forces. Consequently, its melting point is very low (about $$-23\ ^\circ\text{C}$$). There are no free electrons or ions present in either the solid or liquid form, so $$\text{CCl}_{4}$$ is a $$\text{bad conductor}$$ of electricity in both states.

Comparing all four options, only carbon tetrachloride simultaneously satisfies both criteria: $$\text{low melting point}$$ and $$\text{poor electrical conductivity}$$ in solid as well as liquid form.

Hence, the correct answer is Option D.