The correct answer is B) clogged ventilation ducts. Clogged ventilation ducts can cause an open armature connection in a DC propulsion motor by restricting airflow and leading to overheating. This overheating can damage the windings and cause an open circuit in the armature. The other answer choices are not the primary causes of an open armature connection. Excessive starting resistance (A) may contribute to motor issues, but would not directly cause an open armature. A tripped circuit breaker (C) is a symptom, not a cause, of the open armature. Low-load operation (D) could potentially lead to overheating, but clogged ventilation is a more direct cause of the open connection.