Which element consists of positive ions immersed in a “sea” of mobile electrons

Explanation:

Nickel is the only metal listed. Metal elements have metallic bonds between the metal atoms. Positive metal ions in a sea of electrons.

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.