**Core Concept**
Meconium is the first stool of a newborn, composed of materials ingested in utero, such as lanugo, mucus, and intestinal epithelial cells. The composition and passage of meconium are influenced by the fetus's gut motility and the presence of intestinal secretions.

**Why the Correct Answer is Right**
The passage of meconium is generally complete by 48-72 hours after birth. This is because the newborn's gut begins to adapt to the introduction of milk, leading to the production of bile salts and pancreatic enzymes that break down the meconium. The meconium is then gradually replaced by transitional stools, which eventually give way to mature feces. The transition is facilitated by the maturation of the enteric nervous system and the increase in gut motility.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is too early, as the meconium passage is not complete by 24 hours in most newborns.
* **Option B:** This is too late, as the meconium passage is generally complete by 72 hours after birth.
* **Option C:** This is incorrect, as the meconium passage is not influenced by the introduction of solid foods.

**Clinical Pearl / High-Yield Fact**
It's essential to note that delayed passage of meconium can be a sign of intestinal obstruction or other gastrointestinal anomalies, highlighting the importance of monitoring newborns for normal bowel movements and seeking medical attention if there are any concerns.

**Correct Answer: C. 72 hours.**