The correct answer is: A. One-third of the effective span of the T-beam.
The width of the flange of a T-beam should be less than one-third of the effective span of the T-beam. This is to ensure that the beam is able to carry the load without buckling. If the width of the flange is too large, the beam will be more likely to buckle under load.
Option B is incorrect because the distance between the centers of T-beams does not affect the width of the flange.
Option C is incorrect because the breadth of the rib plus twelve times the thickness of the slab is not a factor in determining the width of the flange.
Option D is incorrect because the least of the above is not a valid answer.
The width of the flange of a T-beam is an important factor in the design of a T-beam. It is important to ensure that the width of the flange is not too large, as this can lead to buckling. The width of the flange should be less than one-third of the effective span of the T-beam.