Final answer:
A nylon cord is used to secure the buckles on the MBU-12P straps and buckle assembly, ensuring safety and functionality.
Step-by-step explanation:
The type of cord used to tack down the buckles on the MBU-12P straps and buckle assembly to prevent movement is a nylon cord. Nylon is chosen for its strength, durability, and resistance to environmental factors like UV rays and moisture, which are essential qualities in aviation life support equipment. The process of securing buckles with nylon cord ensures that they remain in place, preventing accidental release or movement that could compromise the safety and functionality of the parachute harness.