**Core Concept**
Child survival index (CSI) is a measure of a country's performance in reducing child mortality. It is a key indicator of the effectiveness of a healthcare system in delivering essential services to children.

**Why the Correct Answer is Right**
The CSI primarily focuses on the reduction of mortality rates in children under five years of age. This includes reducing the risk of death from diseases such as diarrhea, pneumonia, and measles. The index also takes into account the quality of healthcare services, including access to healthcare facilities, healthcare providers, and essential medicines. **Option C: Low birth weight** is a significant risk factor for child mortality and is closely monitored in the CSI.

**Why Each Wrong Option is Incorrect**
**Option A:** Maternal mortality rate is not directly included in the CSI, although it is an important indicator of healthcare quality.
**Option B:** Infant mortality rate is a component of the CSI, but it is not the only factor included in the index.

**Clinical Pearl / High-Yield Fact**
A key component of the child survival index is the reduction of preventable child deaths, which can be achieved through vaccination programs, improved access to healthcare, and education on disease prevention.

**Correct Answer: C. Low birth weight**