The correct answer is: A. Remains within the flange.
A T-beam is a beam that has a flange on one side and a web on the other. The flange is the wider part of the beam, and the web is the narrower part. The neutral axis is an imaginary line that divides the beam into two equal parts, such that the bending stresses on either side of the neutral axis are equal.
If the neutral axis remains within the flange, then the T-beam behaves as a rectangular beam of a width equal to its flange. This is because the flange is the part of the beam that carries most of the bending load. The web, on the other hand, carries very little of the bending load.
If the neutral axis falls below the slab, then the T-beam will not behave as a rectangular beam. This is because the slab will carry some of the bending load. The amount of bending load that the slab carries will depend on the thickness of the slab and the depth of the beam.
If the neutral axis coincides with the geometrical centre of the beam, then the T-beam will behave as a rectangular beam. This is because the flange and the web will carry equal amounts of the bending load.
In conclusion, the correct answer is: A. Remains within the flange.