Substances that are composed of a network of atoms held together by covalent bonds are called O A. ionic lattices
B. giant covalent structures
C. allotropes
D. diatomic networks

giant covalent structures

Explanation:

Covalent solids are made up of atoms joined to one another by covalent bonds to form a giant lattice. The covalent bonds are very strong, this makes the giant covalent solids to be hard and possess high melting points. Covalent solids are nonconductors of electricity due to the absence of free electrons in the structure.

Diamond, graphite and boron nitride are all examples of giant covalent structures.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.