Final answer:
Leslie is using the precipitin test to determine if the blood at the crime scene is human or animal (Option 1). This test is based on antigen-antibody reaction forming a precipitate when human antiserum reacts with human antigens in the blood sample.
Step-by-step explanation:
Leslie, a forensic scientist, is most likely using the precipitin test to determine if a sample is human or animal blood, which corresponds to Option 1. The precipitin test measures the antigen-antibody reaction, specifically when human antiserum is mixed with the blood sample. If the blood is from a human, the reaction will cause a precipitate to form due to the reaction between human antigens and the human antibodies in the antiserum. This test does not provide information about the blood type, whether the substance is blood, or how long the blood has been at the scene. In forensic science, the precipitin test is an important tool when an investigator is trying to establish a link between a blood sample and a potential human source.