Final answer:
In OSHA terms, not being "tied-off" on a roof is considered a Serious violation because there is a high probability of resulting in death or serious harm which the employer should be aware of.
Step-by-step explanation:
When a construction worker is not "tied-off" while working on a roof, it is considered a C) Serious violation according to the Occupational Safety and Health Administration (OSHA).
This type of violation occurs when there is a substantial probability that death or serious physical harm could result from a hazard about which the employer knew or should have known. Since OSHA has clear requirements for fall protection, failing to ensure that workers are properly secured while working at heights can lead to serious or even fatal accidents.