Final answer:
MRI uses radio frequencies and a strong magnetic field to generate images of the brain's structure, while fMRI, a variation of MRI, provides detailed images of brain function. Thus, correct answer is option d.
Step-by-step explanation:
The technique that uses radio frequencies to study the structure of the brain is Magnetic Resonance Imaging (MRI). An MRI machine generates a magnetic field around a patient which causes the hydrogen atoms in the body's cells to align with the field. When the magnetic field is turned off, these hydrogen atoms emit electromagnetic signals as they return to their original positions.
These signals are processed by a computer to create images of the brain's structure. Functional MRI (fMRI) is a variant which measures brain activity by tracking blood flow and oxygen levels, thus providing detailed images of brain function in addition to structure. Both MRI and fMRI are useful for comparing healthy and disordered brain structures.