Electrical conductivity requires mobile charge carriers in both solid and molten states

Steps:

• Identify substances with mobile electrons or ions in solid form for conduction.
• Check if the same mechanism persists when molten, allowing free movement.
• Evaluate alloys: metallic bonding provides delocalized electrons in both states.
• Eliminate options lacking conduction in solid state.

Why A is correct:

• Alloys, as metallic mixtures, conduct via delocalized electrons in solid (per metallic bonding theory) and molten states, where structure remains conductive.

Why the others are wrong:

• B: Hydrocarbons are non-polar covalent compounds with no free electrons or ions, insulating in both states.
• C: Metal oxides are typically ionic insulators in solid form; most do not conduct when molten without dissolution.
• D: Salts are ionic solids that do not conduct due to fixed ions; conduction occurs only when molten, freeing ions.

Final answer: A