The correct answer is B. 15.
The effective span of a simply supported T-beam is the distance between the centerlines of the supports. The overall depth of a T-beam is the distance from the top of the flange to the bottom of the web.
The ratio of the effective span to the overall depth of a simply supported T-beam is limited to 15. This limit is to ensure that the beam is adequately supported and does not deflect excessively.
If the ratio of the effective span to the overall depth is greater than 15, the beam may be more likely to deflect excessively, which could lead to failure.
Option A is incorrect because the ratio of the effective span to the overall depth of a simply supported T-beam is limited to 15, not 10.
Option C is incorrect because the ratio of the effective span to the overall depth of a simply supported T-beam is limited to 15, not 20.
Option D is incorrect because the ratio of the effective span to the overall depth of a simply supported T-beam is limited to 15, not 25.