Final answer:
The first reaction in acid mine drainage and leaching is oxidation of reduced sulfides to sulfate and the release of reduced iron (Fe2+).
Step-by-step explanation:
The first, or initiator, reaction in acid mine drainage and leaching is oxidation of reduced sulfides to sulfate and the release of reduced iron (Fe2+) (option A). In this reaction, reduced sulfides are oxidized to sulfate and Fe2+ ions are released. This process occurs due to the interaction of sulfides with oxidized iron (Fe3+).