Final answer:
Reflective colors are a requirement for life jackets to increase visibility in water and aid in rescue efforts.
Step-by-step explanation:
The requirement for life jackets is reflective colors. Reflective colors on life jackets help increase visibility in dim light or at night, making it easier for rescuers to locate individuals in the water. This requirement ensures that life jackets are effective in promoting safety and preventing accidents.
While other options listed in the question, such as heavyweight material, lack of buoyancy, and inability to adjust size, might be present in some life jackets, they are not specific requirements for all life jackets.