the presence of which reactant is the best indicator of an oxidation - reduction reaction?\nkno3\nh2so4\nnaoh…

Answer:

D. $H_2$

Explanation:

Step1: Understand redox reactions

Oxidation - reduction reactions involve transfer of electrons.

Step2: Analyze each option

$KNO_3$ is a salt, $H_2SO_4$ is an acid, $NaOH$ is a base. These are not characteristic of redox by themselves.

Step3: Consider hydrogen

$H_2$ is often involved in redox. It can be oxidized (lose electrons) or cause reduction (act as a reducing agent) in reactions. For example, in metal - oxide reduction reactions, $H_2$ can reduce metal oxides to metals while getting oxidized itself.