Final answer:
A workplace label is not required when the material is non-hazardous, as the primary purpose of the label is to communicate hazards associated with the substance. The transparency or size of the container does not exempt it from labeling. Specific regulations may vary by country.
Step-by-step explanation:
The question pertains to the circumstances under which a workplace label is not required for a substance. According to health and safety regulations, a label may not be required when the material is non-hazardous. This would make option a) the correct choice. Workplace labels are an essential aspect of hazard communication in the workplace and there are specific scenarios defined by regulations like WHMIS (Workplace Hazardous Materials Information System) in Canada or OSHA's Hazard Communication Standard in the USA, where labels are mandatory. Labels provide critical information about the substance, including potential hazards and necessary precautions.
Labels are typically not required when the material is non-hazardous because the main reason for the label is to communicate the risk. However, the transparency of the container (option b), the size of the container (option c), or the concept that labels are always required (option d) are not automatically exempt conditions from labeling requirements. The details can vary depending on the specific legislation of a country or region.