When may the required label for solid materials be transmitted to the customer?

The correct answer reflects the requirement that safety and hazard information must be conveyed to customers prior to or at the time of initial shipment. This is aligned with OSHA's Hazard Communication Standard, which emphasizes the importance of providing essential information about hazardous materials in a timely manner. By providing labels before shipping or upon initial shipment, companies ensure that users are adequately informed of the hazards associated with the materials they are receiving. This advance communication is crucial for promoting safety and enabling proper handling practices even before the materials are in the customer's possession.

The other options suggest either too late a point in the process or a reactive approach (such as transmitting labels only upon request), which does not comply with the proactive approach mandated by OSHA for hazardous materials communication. Thus, the chosen answer supports the goal of ensuring that all necessary safety information is accessible to protect workers and environments right from the start of the shipping process.