**Core Concept**
The Maternal and Child Health (MCH) program aims to improve the health outcomes of mothers and children. A key indicator for evaluating the success of such a program is a measure that reflects the overall health and well-being of this vulnerable population. This indicator should be sensitive to changes in healthcare services and interventions.

**Why the Correct Answer is Right**
The best indicator for MCH is the Infant Mortality Rate (IMR). IMR is a widely accepted measure of a country's healthcare system and reflects the quality of prenatal, delivery, and postnatal care. It is a sensitive indicator of the overall health status of mothers and children, as it is influenced by various factors such as maternal education, access to healthcare services, and the quality of prenatal and delivery care.

**Why Each Wrong Option is Incorrect**
**Option A:** Maternal Mortality Ratio (MMR) is an important indicator, but it is more specific to maternal health and does not capture the overall health status of children.
**Option B:** Under-5 mortality rate is a broader indicator, but it includes deaths that occur after the neonatal period and may not specifically reflect the quality of prenatal and delivery care.
**Option C:** Fertility rate is not a direct indicator of MCH, as it reflects the number of births per woman rather than the health status of mothers and children.

**Clinical Pearl / High-Yield Fact**
The IMR is a critical indicator of a country's healthcare system, and a low IMR is often considered a benchmark of a well-functioning healthcare system.

**Correct Answer: C. Infant Mortality Rate.**