**Core Concept**
The human body loses a significant amount of water through various routes, including the gastrointestinal tract, kidneys, and respiratory system. In the context of gastrointestinal water loss, the amount of water lost in stools is an essential aspect of understanding fluid balance and electrolyte regulation.

**Why the Correct Answer is Right**
The correct answer is 200ml because the average adult human loses approximately 200ml of water through feces each day. This is primarily due to the water content of ingested food, which is not completely absorbed by the body. The colon absorbs most of the electrolytes and water from the intestinal contents, but a small amount of water is left behind and excreted in the stool. This water loss is balanced by the body's overall water intake, which is primarily obtained through drinking fluids and metabolizing food.

**Why Each Wrong Option is Incorrect**
**Option A:** 50ml is an underestimation of the average water loss through stools. This amount is closer to the daily water loss through sweat, but not through feces.
**Option C:** 300ml is an overestimation of the average water loss through stools. While individual values can vary, 300ml is significantly higher than the average daily loss.
**Option D:** 350ml is also an overestimation of the average water loss through stools. This option is even higher than option C, making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the body's water balance is tightly regulated, and even small changes in fluid intake or loss can have significant effects on electrolyte balance and overall health. This is particularly important in clinical settings where patients may experience fluid imbalances due to various medical conditions or treatments.

**✓ Correct Answer: B. 200ml**