**Core Concept**
The literacy rate is a crucial indicator of a population's educational status. Calculating literacy rates involves determining the proportion of individuals who can read and write in a given population. The denominator used in the calculation is essential, as it affects the overall literacy rate.

**Why the Correct Answer is Right**
The correct answer is **C.** "Population aged 7 years and above who are not disabled". The denominator used in the 2001 census for calculating the literacy rate was the population aged 7 years and above who are not disabled. This is because the literacy rate is typically calculated for the population that is capable of reading and writing, excluding those who are disabled or below the age of 7. The use of this denominator ensures that the literacy rate accurately reflects the educational status of the population.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect as it is not a specific or relevant denominator for calculating the literacy rate.
* **Option B:** is incorrect as it refers to the population aged 7 years and above, but does not account for those who are disabled.
* **Option D:** is incorrect as it refers to the population of a specific age group, but does not account for those who are disabled.

**Clinical Pearl / High-Yield Fact**
When calculating literacy rates, it's essential to use a denominator that accurately reflects the population's educational status. In this case, the population aged 7 years and above who are not disabled is the most suitable denominator.

**Correct Answer:** C. Population aged 7 years and above who are not disabled.