**Core Concept**
The investigation of fat malabsorption involves assessing the digestion and absorption of fats in the gastrointestinal tract. Fat malabsorption can be caused by various factors, including pancreatic insufficiency, bile salt deficiency, and gastrointestinal disorders. To diagnose the underlying cause, several tests can be employed to evaluate the digestion and absorption of fats.

**Why the Correct Answer is Right**
Investigation 1 of fat malabsorption typically involves a 72-hour fecal fat collection test, which measures the amount of fat excreted in the stool over a period of 3 days. This test helps to determine if there is an excessive loss of fat in the stool, indicating malabsorption. The 72-hour fecal fat collection test is a direct measure of fat malabsorption and is often used as the first line of investigation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the investigation of fat malabsorption.
**Option B:** This option is not a direct measure of fat malabsorption and is not typically used as the first line of investigation.
**Option C:** This option is not used for investigating fat malabsorption; it is used for other gastrointestinal disorders.

**Clinical Pearl / High-Yield Fact**
The 72-hour fecal fat collection test is a sensitive and specific test for fat malabsorption. However, it requires careful collection and analysis of stool samples over a prolonged period, which can be challenging for patients.

**Correct Answer:** C.