When a metallic element is converted to a positive ion during a chemical reaction with oxygen, the element is said to be?

The metal is being oxidized.

Metals- along with other elements- are neutral and have oxidation state of in their respective elementary states.

Metal atoms can lose electrons to electronegative elements such as oxygen on exposure. Atoms that have lost one or more negatively-charged electrons are no longer neutral and demonstrate a positive charge. Those atoms have converted to positively charged ions or cations.

Oxidation is the process of losing electrons whereas Reduction refers to the process of gaining electrons. "OIL RIG." Metal atoms here lose electrons and are thus oxidized.

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.