**Core Concept**
The question is testing the minimum volume of gastrointestinal (GI) bleeding required to cause melena, a type of tarry stools. Melena is a clinical manifestation of upper GI bleeding, and its occurrence is influenced by the amount and location of bleeding.

**Why the Correct Answer is Right**
The correct answer is 60 ml because it is the minimum volume of bleeding that can cause melena. This is due to the presence of intestinal mucosal enzymes, such as gastric and duodenal mucosal enzymes, which start to break down blood as soon as it enters the small intestine. When the bleeding is minimal (less than 60 ml), the blood is quickly broken down by these enzymes, resulting in a small amount of digested blood products being absorbed and excreted in the stool. However, when the bleeding exceeds 60 ml, the amount of undigested blood products in the stool increases, resulting in the characteristic black, tarry appearance of melena.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 ml is too small to cause melena, as the blood would be quickly broken down by intestinal enzymes, and only a small amount of digested blood products would be absorbed and excreted in the stool.

**Option B:** 40 ml is still relatively small and would likely result in occult bleeding, which is not visible to the naked eye and may not cause melena.

**Option D:** 115 ml is a large amount of bleeding and would likely cause hematemesis (vomiting of blood) in addition to melena.

**Clinical Pearl / High-Yield Fact**
In clinical practice, melena is often a sign of significant upper GI bleeding, and its presence should prompt a thorough investigation to identify the underlying cause, including endoscopy, imaging studies, and laboratory tests.

**✓ Correct Answer: C. 60 ml**