**Core Concept**
Literacy rate is a crucial public health indicator that measures the ability of a population to read and write. It is an essential component of the overall development of a country, reflecting the level of education and socio-economic status of its citizens.

**Why the Correct Answer is Right**
The parameter taken into account for calculating the literacy rate is the ability of individuals to read and write with understanding a simple message in their language. This is often measured using a functional literacy test that assesses basic skills in reading and writing, such as recognizing letters, words, and short sentences. The test may also include questions on basic arithmetic and comprehension to evaluate the individual's ability to apply literacy skills in everyday life.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as this option is missing.
* **Option B:** Not correct, as the parameter taken into account is not the ability to communicate in a foreign language, but rather the ability to read and write in the individual's native language.
* **Option C:** Not correct, as the parameter taken into account is not the ability to perform mathematical calculations, but rather the ability to read and write with understanding.

**Clinical Pearl / High-Yield Fact**
When calculating the literacy rate, it's essential to use a functional literacy test that assesses basic reading and writing skills, rather than a test that measures advanced literacy or language proficiency.

**Correct Answer: C. Ability to read and write with understanding a simple message in their language.**