What is a primary source of cyanide found in wastewater?

The correct answer is etching baths, as they are a primary source of cyanide in wastewater. In various industrial processes, particularly in metal finishing and electroplating, cyanide compounds are frequently used in etching baths to help dissolve metals. The etching process uses cyanide solutions to create desired surface finishes on metals, providing both aesthetic and functional properties.

When the contaminants from these baths are not properly treated or managed, they can end up in wastewater, posing significant environmental hazards. Cyanide is highly toxic, and its presence in wastewater necessitates careful monitoring and treatment to prevent harm to aquatic ecosystems and human health.

The other options, while potentially related to wastewater issues, do not primarily contribute cyanide. Excess fertilizers can lead to nutrient pollution but do not contain cyanide. Rainwater runoff can carry various pollutants but is not a significant source of cyanide. Food processing may introduce different organic and inorganic waste but typically doesn't involve cyanide unless specific food products are processed that might utilize it. Thus, etching baths stand out as the main industrial source of cyanide contamination in wastewater.