What conclusion is best supported by the decay chain?
each alpha decay results in an isotope that is slightly more stable than the original isotope.
each beta decay results in an isotope that is slightly more stable than the original isotope.
the atomic number increase caused by two beta decays is the same as the decrease caused by one alpha decay.
the mass number increase caused by two beta decays is the same as the decrease caused by one alpha decay.

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.

c. 75.7

i think that is the right