**Core Concept**
The question requires an understanding of methods used to estimate the infant population in a given area. One such method is the "Number of Children Under 5" (NCU5) technique, also known as the "Under 5" method. This method relies on the fact that approximately 70-80% of children under the age of 5 are under the age of 2, and about 50% of children under the age of 5 are under 1 year old.

**Why the Correct Answer is Right**
The Under 5 method is a quick and simple technique used to estimate the infant population. By counting the number of children under 5, the infant population can be estimated by considering the proportion of children under 2 and 1 year old. This method is particularly useful in areas where a full census is not feasible. The estimation is based on the assumption that the age distribution of children under 5 is relatively stable across different populations.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incomplete and does not specify a valid method for estimating the infant population.
**Option B:** This option is not recognized as a valid method for estimating the infant population.
**Option C:** This option is not a recognized method for estimating the infant population.

**Clinical Pearl / High-Yield Fact**
The Under 5 method is a simple and effective way to estimate the infant population in areas where a full census is not feasible. This method can be used in conjunction with other demographic techniques to provide a more accurate estimate of the population.

**Correct Answer: C. Under 5 method.**