Which is NOT considered a hazardous substance?

Stagnant ditch water is not considered a hazardous substance in the context of this question because it typically does not contain harmful chemicals or toxins like the other options listed. While stagnant water can pose environmental health risks, such as harboring bacteria or insects, it is not classified as a hazardous substance under most regulatory standards like the other choices.

On the other hand, oily rags, used fuel filters, and lead paint chips are all considered hazardous due to their potential to cause harm. Oily rags can catch fire easily and may contain flammable materials. Used fuel filters can hold residues of petroleum products, which are toxic. Lead paint chips are hazardous because lead is a toxic heavy metal that can cause serious health issues. These materials are regulated to prevent environmental contamination and protect public safety, underscoring why stagnant ditch water does not fall into the same category.