ANS:D - least of the above.

1. One-sixth of the effective span: This criterion sets a limit on the width of the flange based on the effective span of the beam. One-sixth of the effective span is a relatively arbitrary value and might not be directly related to the structural behavior or design considerations specific to L-beams.
2. Breadth of the rib + four times thickness of the slab: This criterion considers the dimensions of the rib and the thickness of the slab. The width of the flange is limited by the sum of the breadth of the rib and four times the thickness of the slab. This approach accounts for the structural configuration of the beam and the surrounding elements but may not be universally applicable to all L-beams.
3. Breadth of the rib + half clear distance between ribs: This criterion involves the breadth of the rib and half the clear distance between ribs. It accounts for the spacing between ribs and the structural configuration of the beam. This approach ensures that the width of the flange does not exceed the available space between adjacent ribs, preventing interference and ensuring proper construction.

Considering these options, the third option, breadth of the rib + half clear distance between ribs, appears to be the most comprehensive and practical criterion for limiting the width of the flange of an L-beam. It accounts for both the dimensions of the rib and the spacing between adjacent ribs, which are critical factors in determining the dimensions of the flange while avoiding interference and ensuring structural integrity. Therefore, this option is likely the most appropriate recommendation for specifying the width of the flange in L-beams.