To determine when a transformer-supplied system must be grounded if the supply voltage to the transformer exceeds a certain value, we should review the requirements regarding the grounding of electrical systems.
In general, the need for grounding arises to ensure safety by preventing electrical shock and protecting the system from over-voltage conditions.
Given the problem, we want to find the supply voltage threshold at which grounding becomes necessary for a system operating at less than 50V. Based on safety regulations and standards, the specific voltage limit after which grounding is required can be identified.
From the provided data, if the supply voltage to the transformer exceeds 150V to ground, the transformer-supplied system indeed must be grounded to ensure safety and compliance with electrical codes.
Hence, the correct choice is:
O d. 150V to ground
