**Core Concept**
Meckel's diverticulum is a congenital anomaly resulting from the incomplete closure of the vitelline duct, leading to the formation of a small, true diverticulum in the ileum. This condition can cause various complications, including intestinal obstruction, bleeding, and diverticulitis.

**Why the Correct Answer is Right**
The most common presentation of Meckel's diverticulum is gastrointestinal bleeding, often manifesting as painless, bright red rectal bleeding in children. This occurs due to the presence of ectopic gastric mucosa within the diverticulum, which secretes acid and leads to mucosal ulceration and bleeding. The ectopic gastric mucosa can also be a site for peptic ulceration, further contributing to bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** While Meckel's diverticulum can cause intestinal obstruction, this is a less common presentation, typically occurring in adults or in cases of large diverticula.
* **Option B:** Diverticulitis is a possible complication of Meckel's diverticulum, but it is not the most common presentation.
* **Option D:** Intussusception is a possible complication of Meckel's diverticulum, but it is not the most common presentation.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is a common congenital anomaly, present in approximately 2% of the population, and is often asymptomatic. However, when it presents, the most common symptom is painless gastrointestinal bleeding, making this a key consideration in the differential diagnosis of lower gastrointestinal bleeding in children.

**Correct Answer:** C.