Final answer:
A substrate control from a crime scene involving fire is an uncontaminated sample collected from an area near the point of origin, serving as a reference for comparison during analysis.
Step-by-step explanation:
A substrate control is a sample taken from an area near the point of origin of a fire in a crime scene. It serves as a comparison sample for the analysis of potential accelerants. The substrate control is typically collected from an area that was not directly affected by the fire, so it will be uncontaminated by the fire itself.
In a crime scene involving fire, a substrate control is not likely to be covered in accelerant (option A), as accelerants are typically found in areas where the fire was intentionally started. Additionally, the substrate control is not the cause of the fire (option D), but rather a reference sample for comparison. However, it is likely to be near the point of origin (option C), as it is important to collect a sample from as close to the source of the fire as possible.