The correct answer is A. 1.5.
A deep beam is a beam whose ratio of effective span to overall depth (1/D) is less than 1.5. This means that the depth of the beam is greater than or equal to 1.5 times its effective span. Deep beams are more susceptible to shear and bending stresses than shallow beams, and they require special design considerations.
Option B is incorrect because the ratio of effective span to overall depth (1/D) for a deep beam is less than 1.5, not greater than 2.0.
Option C is incorrect because the ratio of effective span to overall depth (1/D) for a deep beam is less than 1.5, not greater than 2.5.
Option D is incorrect because the ratio of effective span to overall depth (1/D) for a deep beam is less than 1.5, not greater than 3.0.