Final answer:
To test an unknown substance in the field, a chemical reagent is used depending on the substance being tested.
Step-by-step explanation:
The tool used to test an unknown substance in the field to determine its composition or to identify it can vary depending on the type of substance. Reagents are chemicals that cause a reaction with specific types of substances and are often used for this purpose.
The answers listed, A) Marquis reagent, B) Mandelin reagent, C) Scott reagent, and D) Duquenois-Levine reagent, are all chemical reagents used for different identification tests. The Marquis reagent is commonly used to test for the presence of alkaloids; the Mandelin reagent is used for various substances including ketamine and PMA; the Scott reagent is specifically for cocaine; and the Duquenois-Levine reagent is usually employed in law enforcement for the presumptive identification of marijuana.
When answering a question about which tool is used to test an unknown substance, it is important to know the context in which the substance is being tested. Without more specific information, we cannot definitively choose one of the reagents listed. However, the Duquenois-Levine reagent is a well-known field test for marijuana, which is commonly tested for in the field by law enforcement.