The correct answer is: A. One-sixth of the effective span.
The width of the flange of an L-beam should be less than one-sixth of the effective span to ensure that the beam is adequately supported. If the width of the flange is too great, the beam may not be able to support the load it is designed to carry.
Option B is incorrect because the breadth of the rib + four times thickness of the slab is not a factor that is considered when determining the width of the flange of an L-beam.
Option C is incorrect because the breadth of the rib + half clear distance between ribs is not a factor that is considered when determining the width of the flange of an L-beam.
Option D is incorrect because the least of the above is not a factor that is considered when determining the width of the flange of an L-beam.