* Cubic Solid: Diamond has a crystal structure that is cubic.

* Poor Electrical Conductor in State: Diamond is a non-metal and has a strong covalent network structure. This means electrons are tightly bound and cannot move freely, making it a poor conductor of electricity.

* Good Conductor when Fused: While diamond is a very hard and high-melting point material, it doesn't melt easily. Even if it did, the molten state would still be a poor conductor due to the strong covalent bonds.

Let's analyze the other options:

* (a) Sodium chloride (NaCl): This is an ionic compound, forming a cubic crystal structure. While it's a good conductor when molten (the ions are free to move), it's a poor conductor in its solid state.

* (c) Sulfur: Sulfur exists in various allotropes, some of which can be crystalline. However, it's generally a poor conductor in both solid and molten states.

* (d) Chromium: Chromium is a metal and is an excellent conductor of electricity in both solid and molten states.

Therefore, only diamond fits all the criteria.