Final answer:
The reddish-brown substance in rat tears and saliva is called porphyrin, which is not listed in the options given, and is different from hemoglobin, melanin, insulin, and serotonin that serve other functions in the body.
Step-by-step explanation:
The reddish-brown substance found in the tears and saliva of rats is called porphyrin. This compound, which is not one of the options listed in the question, can sometimes be mistaken for blood but it is actually a pigment that rats naturally produce. It is often associated with stress or illness in these animals. Neither hemoglobin, melanin, insulin, nor serotonin is the correct answer to the student's question, as these substances serve different functions in the body. For instance, hemoglobin is found in red blood cells and is responsible for carrying oxygen, melanin provides pigment to the skin and hair, insulin is a hormone that regulates blood sugar levels, and serotonin is a neurotransmitter with many functions in the brain and body, as shown in studies like Steinbusch H.W.'s research on the distribution of serotonin in the rat's central nervous system.