Final answer:
In the NFPA 705 marking system, the red color indicates reactivity.
Step-by-step explanation:
In the NFPA 705 marking system, the red color indicates reactivity. The NFPA 705 marking system is used to quickly identify the hazards of a chemical substance. The yellow diamond in the system represents reactivity hazards, indicating how readily the substance will undergo detonation or a violent chemical change. Therefore, the correct answer is a. Red.