The correct answer is D. All of the above.
The width of the flange of a T-beam, which may be considered to act effectively with the rib, depends upon the breadth of the rib, the overall thickness of the rib, and the centre to centre distance between T-beams.
The breadth of the rib is the width of the horizontal part of the T-beam. The overall thickness of the rib is the total thickness of the rib, including the flange. The centre to centre distance between T-beams is the distance between the centres of two adjacent T-beams.
The width of the flange is important because it determines the amount of load that the flange can carry. The overall thickness of the rib is important because it determines the strength of the rib. The centre to centre distance between T-beams is important because it determines the amount of lateral support that the flange receives from the ribs.
If the width of the flange is too small, the flange will not be able to carry the load. If the overall thickness of the rib is too small, the rib will not be strong enough. If the centre to centre distance between T-beams is too large, the flange will not receive enough lateral support and will be more likely to buckle.
Therefore, the width of the flange of a T-beam, which may be considered to act effectively with the rib, depends upon the breadth of the rib, the overall thickness of the rib, and the centre to centre distance between T-beams.