**Explanation:**

The correct answer is option 'B', which states that a census is conducted every 10 years.

A census is a systematic and comprehensive collection of data about the population of a country or a specific region. It aims to gather information about various demographic, social, and economic characteristics of the population.

**Reasons for conducting a census:**

1. **Population Count:** One of the primary reasons for conducting a census is to accurately count the population. This information is essential for planning purposes, such as resource allocation, infrastructure development, and policy formulation.

2. **Demographic Analysis:** A census provides valuable information about the demographic composition of the population, including age, sex, marital status, and educational attainment. This data helps in understanding population trends, identifying disparities, and formulating appropriate policies.

3. **Social and Economic Characteristics:** Census data also collects information about social and economic characteristics, such as occupation, income, housing, and household size. This information is crucial for assessing the well-being of the population, identifying areas of social inequality, and formulating targeted interventions.

4. **Redistricting and Electoral Representation:** Census data is used for redistricting, which involves the realignment of electoral boundaries based on population changes. It ensures that each district has a roughly equal number of residents, ensuring fair and equitable representation in elections.

5. **Policy Formulation:** Census data serves as a foundation for evidence-based policy formulation. Governments, researchers, and organizations use census data to identify social, economic, and demographic trends, which inform the development of policies and programs.

**Frequency of Census:**

Census is conducted at regular intervals to ensure the accuracy and relevance of the data. The frequency of conducting a census varies from country to country, depending on various factors such as population size, resources, and data requirements.

In most countries, including the United States, India, and many others, a census is conducted every 10 years. This interval allows for significant changes and trends in the population to be captured, while also providing enough time for data analysis and policy formulation.

By conducting a census every decade, governments can track population growth, migration patterns, changes in social and economic characteristics, and make informed decisions based on the collected data.

In conclusion, a census is conducted every 10 years to gather comprehensive and accurate data about the population, which is essential for planning, policy formulation, and ensuring fair representation in electoral processes.