Final answer:
Ultrasounds are the safest imaging study during pregnancy since they do not use ionizing radiation, unlike CT scans, PET scans, and X-rays.
Step-by-step explanation:
Among the imaging studies listed, ultrasounds can be used most safely during pregnancy. Ultrasounds are the least invasive of imaging techniques and use no electromagnetic radiation, making them ideal for monitoring fetal development. In contrast, CT scans, PET scans, and X-rays involve ionizing radiation, which may pose a risk to both the mother and developing fetus and should be used cautiously.