When the length of the steel wire is doubled from 75 cm to 150 cm, the stretch caused by hanging the same equipment doubles as well, resulting in a stretch of 8.00 mm.
The problem at hand involves the physical principle of elasticity in materials, which describes how a material deforms under load and then returns to its original shape when the load is removed. Given that a 125-kg piece of equipment stretches a 75 cm steel wire by 4.00 mm, we need to ascertain the stretch when the wire's length is doubled, assuming the material's elasticity properties remain consistent.
The deformation in such uniform materials is directly proportional to the length when all other factors remain constant. Therefore, if the length of the wire is doubled, the stretch would also double. In this case, the stretch would be 8.00 mm for a 150 cm wire.
Hence, the correct answer is b. 8.00 mm.