Which of the following is a presumptive chemical used for screening blood at a crime scene?

Leucomalachite green is a presumptive chemical commonly used in screening tests for blood at crime scenes. It interacts with hemoglobin, the protein in red blood cells that carries oxygen, and produces a color change that indicates the potential presence of blood. This characteristic makes it a valuable tool for crime scene analysts when they need to identify suspected bloodstains.

Phenolphthalein and Benzidine were also historically used for similar purposes. However, they are either less sensitive or have fallen out of favor due to safety concerns and the availability of more reliable alternatives. Blue Star, while useful as a luminescent reagent for detecting blood, is not a chemical compound but rather a product that indicates the presence of blood under certain conditions. Therefore, Leucomalachite green remains the preferred chemical for presumptive blood tests in many forensic applications.