Final answer:
The unit for measuring gradient strength (amplitude) in a magnetic field is millitesla per meter (mT/m), which describes how much the magnetic field changes per unit distance.
Step-by-step explanation:
The question asks about the unit of measurement for gradient strength (amplitude) in the context of a magnetic field. Gradient strength in magnetic fields is usually measured in millitesla per meter (mT/m). This unit specifies how much the magnetic field changes per unit of distance. The SI unit for magnetic field strength B is the tesla (T), but when discussing gradients, we refer to changes in the field strength over a distance, thus mT/m is used. It's important to note that Tesla (T) is also used to measure the overall strength of magnetic fields, so choice B would also relate to magnetic fields if it was referring to the general field strength and not specifically the gradient.