The correct answer is B) uninsulated wire. On a DC motor brush rigging, the "pigtail" refers to the short, uninsulated wire that connects the brush to the brush holder. This pigtail allows electrical current to flow from the commutator to the brush, powering the motor. The other options are incorrect because: A) The brush holder is the component that holds the brush in place, but it is not the pigtail itself. C) The flexible spring adjuster is a separate component that applies tension to the brush, but it is not the pigtail. D) The feather spring is also a separate component that helps maintain contact between the brush and the commutator, but it is not the pigtail.