The maximum voltage generated by a single loop DC generator occurs when the conductor is cutting the magnetic lines of force at an angle of:

In a single loop direct current (DC) generator, the maximum voltage is generated when the conductor is effectively maximizing its interaction with the magnetic field. This occurs when the conductor is oriented at 90 degrees to the magnetic lines of force.

When the conductor cuts through the magnetic field lines at this angle, it experiences the greatest rate of change of magnetic flux, according to Faraday's law of electromagnetic induction. This law states that the induced electromotive force (emf) in a loop is directly proportional to the rate of change of magnetic flux through the loop. At 90 degrees, the conductor has the most direct interaction with the field, leading to the highest induced voltage.

In contrast, at 0 degrees and 180 degrees, the conductor is parallel to the magnetic lines of force, resulting in no cutting of the lines and therefore no voltage generation. At 45 degrees, although there is some interaction, it does not yield as much induced voltage as at 90 degrees, since the alignment is not optimal. Thus, the correct answer reflects the angle that allows for the maximum electromagnetic induction effect, which is indeed 90 degrees.