The correct answer is D. all of the above.
The flux distribution in a DC generator is determined by the size of the air gap, the shape of the pole shoe, and the clearance between tips of the adjacent pole shoes.
The size of the air gap is the distance between the stator and rotor. The larger the air gap, the weaker the magnetic field. The shape of the pole shoe is the curved surface that directs the magnetic field. The shape of the pole shoe affects the strength of the magnetic field. The clearance between tips of the adjacent pole shoes is the distance between the two pole shoes. The smaller the clearance, the stronger the magnetic field.
The flux distribution in a DC generator is important because it affects the output voltage of the generator. The greater the flux distribution, the greater the output voltage.