The correct answer is A) radial mount. In a bi-directional motor, the brush holder needs to be able to accommodate the motor's ability to rotate in both directions. A radial mount brush holder is the appropriate choice, as it allows the brushes to be positioned symmetrically around the commutator, enabling the motor to operate effectively in either direction. The other options are incorrect because: B) a trailing mount would only work for one direction of rotation, C) a leading mount would only work for the opposite direction, and D) using both leading and trailing mounts would not be necessary or recommended for a bi-directional motor.