Solid crystals with bonds are the best insulators.

The given blank can be filled with covalent.

Explanation:

Insulators refer to the substances, which restrict the flow of electrical current.  A covalent bond refers to a chemical bond, which includes the sharing of electron pairs between the atoms. The covalent solids are a category of elongated-lattice compounds in which every atom is bonded covalently with its closest neighbors. In the solid crystals with covalent bonds, due to the absence of delocalized electrons, the covalent solids do not conduct electricity. Thus, the solid crystals with covalent bonds are the best insulators.

plasma is the state of matter where the substance is ionized and has no definite shape or volume. the fact of being ionized makes the substance a good electrical conductor.

the answer is barium and copper bromide