**Core Concept**
The investigation of choice for a pregnant lady with an upper abdominal mass requires consideration of safety for both the mother and the fetus, ideally using non-ionizing radiation. **Magnetic Resonance Imaging (MRI)** and **Ultrasound** are preferred over **Computed Tomography (CT)** scans due to the lack of ionizing radiation.

**Why the Correct Answer is Right**
Given the options are not explicitly stated, the general approach would favor **Ultrasound** as the initial investigation of choice for an upper abdominal mass in a pregnant woman. This is because ultrasound is widely available, does not use ionizing radiation, and can provide valuable information about the nature of the mass.

**Why Each Wrong Option is Incorrect**
**Option A:** If this were CT, it would be incorrect due to the use of ionizing radiation, which is generally avoided in pregnancy unless absolutely necessary.
**Option B:** If this were MRI, while MRI is safe in pregnancy, it's typically reserved for cases where ultrasound is inconclusive, due to cost and availability considerations.
**Option D:** If this were another imaging modality involving ionizing radiation, it would also be incorrect for the same reasons as CT.

**Clinical Pearl / High-Yield Fact**
In pregnant women, **Ultrasound** is the first-line imaging modality for assessing abdominal masses due to its safety profile and effectiveness. It's crucial to minimize exposure to ionizing radiation during pregnancy.

**Correct Answer:** Correct Answer: B. Ultrasound.