## Core Concept
The question pertains to the comparative radiation exposure associated with various diagnostic imaging modalities during pregnancy. Understanding the relative radiation doses from different imaging techniques is crucial for minimizing fetal exposure and potential risks.

## Why the Correct Answer is Right
Ultrasound (option ) is widely recognized for its safety during pregnancy due to its non-invasive nature and the absence of ionizing radiation. It uses high-frequency sound waves to create images of the fetus and is the preferred initial imaging modality for pregnant women. In contrast, X-ray (option A), CT scans (option B), and nuclear medicine scans (option D) involve ionizing radiation, which poses potential risks to the developing fetus, including increased risk of childhood cancer and genetic mutations.

## Why Each Wrong Option is Incorrect
- **Option A:** X-rays involve ionizing radiation, which can penetrate tissues and potentially harm the fetus, especially during organogenesis.
- **Option B:** CT scans use X-rays to create detailed cross-sectional images of the body and expose patients to a higher dose of radiation compared to conventional X-rays, increasing the risk to the fetus.
- **Option D:** Nuclear medicine scans involve the administration of radioactive tracers, which can also expose the fetus to ionizing radiation, although the doses can vary widely depending on the specific procedure and tracer used.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while no imaging modality is completely risk-free during pregnancy, ultrasound is generally considered the safest due to its lack of ionizing radiation. When choosing an imaging modality for a pregnant patient, the benefits of the test must be weighed against the potential risks, especially in terms of radiation exposure.

## Correct Answer: C. Ultrasound.