Which of these is an indication that a chemical reaction has occurred?
a. formation of gas
b. freezing of a substance
c. melting of a substance
d. boiling of a substance?

A. formation of a gas
The rest of these processes can be reversed easily; however, a chemical reaction cannot be reversed (unless it is a reversible one) so easily. For example, if you burn paper and release carbon dioxide, putting the ashes and the gas in the same container and cooling will not once again produce paper.
Melting, boiling and freezing can be easily reverted. 

Process which indicates that a chemical reaction has occurred is a. formation of gas. It is the very first evidence which shows that reaction has just happened.