**Core Concept**
In the context of social and preventive medicine, literacy rate is a crucial indicator of a population's educational and socio-economic status. The census literacy rate is a measure of the percentage of a population that can read and write at a basic level.

**Why the Correct Answer is Right**
The correct answer is based on the definition of literacy, which is the ability to read and write with understanding. This definition is widely accepted by international organizations, such as the United Nations, and is used as a standard measure of literacy rates. The ability to read and write is a fundamental skill that enables individuals to access information, communicate effectively, and participate in their community. In the census, literacy rate is assessed by asking individuals if they can read and write, and this information is used to calculate the percentage of the population that is literate.

**Why Each Wrong Option is Incorrect**

* **Option A:** Attended literacy classes for one year is not a reliable measure of literacy rate, as attending classes does not necessarily mean that an individual has acquired the ability to read and write. Many factors, such as the quality of instruction and the individual's prior knowledge, can influence the effectiveness of literacy classes.
* **Option B:** Ability to write signature is not a valid measure of literacy rate, as it only assesses the ability to write one's name and does not evaluate the ability to read and write with understanding. Many individuals may be able to write their signature but lack basic reading and writing skills.

**Clinical Pearl / High-Yield Fact**
When assessing literacy rate, it's essential to remember that literacy is a continuum, and individuals may exhibit varying levels of literacy skills. Therefore, a simple yes or no question about reading and writing may not capture the full range of literacy skills within a population.

**✓ Correct Answer: C. Ability to read and write**