If a self-excited DC generator running at rated speed fails to deliver an output voltage, which of the following conditions may be a cause of this difficulty?

Correct Answer: Option B — An open shunt-field circuit.

Self-excited direct current generators depend on residual magnetism in the main poles to initiate field current buildup through the shunt circuit. An open shunt-field circuit prevents this regenerative buildup entirely, resulting in zero terminal voltage output at rated speed.