**Core Concept**
The child survival index, also known as the Child Survival Index (CSI), is a measure used to evaluate the health and well-being of children in a given population. It is a key indicator of the effectiveness of public health programs and policies aimed at reducing child mortality.

**Why the Correct Answer is Right**
The Child Survival Index specifically measures the percentage of children surviving to the age of 5 years. This age is chosen because it is a critical period of childhood when children are most vulnerable to infectious diseases and other health threats. The CSI is calculated by dividing the number of children surviving to age 5 by the number of children born, and then multiplying by 100 to express the result as a percentage. The CSI is an important tool for monitoring progress towards the United Nations' Sustainable Development Goal (SDG) 3, which aims to reduce child mortality rates.

**Why Each Wrong Option is Incorrect**
* **Option A:** The percentage of children surviving to age 1 year is a different measure known as the Neonatal Survival Index (NSI).
* **Option B:** The percentage of children surviving to age 10 years is not a standard measure used in public health, although it may be used in specific research studies or contexts.
* **Option C:** The percentage of children surviving to age 5 years is the correct measure, so this option is not a distractor.

**Clinical Pearl / High-Yield Fact**
The Child Survival Index is a crucial indicator of the effectiveness of public health programs, particularly those aimed at reducing child mortality rates. It is essential to track progress towards this goal to ensure that children receive the healthcare and support they need to survive and thrive.

**Correct Answer: C. 5 years.**