Final answer:
c) Ultrasound
For suspecting kidney stones in a pregnant woman, an ultrasound is the safest diagnostic tool because it avoids exposing the mother and fetus to ionizing radiation and is less invasive.
Step-by-step explanation:
If you suspect that a pregnant woman has a kidney stone, the safest and most appropriate diagnostic tool to use is an ultrasound. This imaging technique is the least invasive and does not involve exposing the patient to ionizing radiation, which is especially important during pregnancy to avoid potential risks to the developing fetus. While X-rays and CT scans are commonly used to diagnose renal calculi in the general population, these methods involve radiation and are generally not the first choice for pregnant women. MRI can also be considered safe during pregnancy; however, it may not be as readily available or as efficient in detecting kidney stones as an ultrasound.