**Core Concept**
Fat malabsorption can be investigated through various tests that assess the digestion and absorption of fats in the gastrointestinal tract. The underlying principle involves measuring the amount of fat excreted in the stool or assessing the products of fat digestion and absorption.

**Why the Correct Answer is Right**
Since the correct answer is not specified, let's discuss the common investigations for fat malabsorption. Tests like **72-hour fecal fat test** measure the amount of fat in the stool, indicating malabsorption. Other tests include **serum beta-carotene** and **vitamin A and E levels**, as these fat-soluble vitamins are affected by fat malabsorption.

**Why Each Wrong Option is Incorrect**
**Option A:** Without knowing the specific option, typically, a test not used for investigating fat malabsorption would be one that does not directly measure fat or its by-products in the stool, blood, or another relevant bodily fluid.
**Option B:** Similarly, if this option is unrelated to the measurement of fat digestion or absorption, it would be incorrect for investigating fat malabsorption.
**Option C:** This option would be incorrect if it does not involve assessing the products of fat digestion or the absorption of fats.
**Option D:** This would be the correct answer if it represents a test or method not used for investigating fat malabsorption.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **fat malabsorption** can lead to deficiencies in **fat-soluble vitamins** (A, D, E, and K), which can have significant clinical implications.

**Correct Answer:** D. Upper GI endoscopy