**Core Concept**
Low birth weight is a critical indicator of neonatal health, defined by the World Health Organization (WHO) as a birth weight less than 2,500 grams. This definition encompasses various underlying factors, including intrauterine growth restriction, preterm birth, and maternal health conditions. The exact weight threshold is crucial for guiding neonatal care and predicting long-term outcomes.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's definition, which takes into account the average weight of full-term newborns worldwide. This threshold is significant because low birth weight is associated with increased risks of neonatal mortality, respiratory distress, and other complications. The exact weight of 2,500 grams is chosen to distinguish between normal and low birth weight, allowing for targeted interventions and improved outcomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not align with the WHO's definition of low birth weight. The exact threshold is crucial for accurate diagnosis and management.
* **Option B:** This option is incorrect because it is too high to be considered low birth weight. Newborns weighing 2,500-3,000 grams are generally considered to be at a normal birth weight.
* **Option C:** This option is incorrect because it is too low to be considered low birth weight. Newborns weighing less than 1,500 grams are typically classified as very low birth weight, which carries a higher risk of complications.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that low birth weight is a critical indicator of neonatal health and that the exact weight threshold of 2,500 grams is a key factor in guiding neonatal care and predicting long-term outcomes.

**Correct Answer: D. 2,500 grams**