The presence of which reactant is the best indicator of an oxidation-reduction reaction? kno3 h2so4 naoh h2

The correct answer is H2.
An indicator of an oxido-reduction reaction can be defined as an indicator which undergoes a definite color change at a specific electrode potential. Almost all redox indicators with true organic redox systems involve a proton as a participant in their electrochemical reaction (for example, methylene blue).  

The mass has escaped into a gas which you can not see so the law stays true.