The correct answer is option 'A' - Census inquiry.

A census inquiry is a method of data collection in which information is gathered from every individual or unit in a population. It involves collecting data from each member of the population rather than from a sample. This complete enumeration allows for a comprehensive and accurate representation of the population being studied.

There are several reasons why a census inquiry may be conducted:

1. Accuracy: By collecting data from every member of the population, a census inquiry ensures a high level of accuracy in the information collected. It eliminates the potential for sampling errors or biases that may occur in sample inquiries.

2. Small populations: Census inquiries are particularly useful for small populations where it is feasible to collect data from every individual or unit. In such cases, a sample inquiry may not be necessary or practical.

3. Specific information: In some cases, the researcher may require specific information about each individual or unit in the population. A census inquiry allows for the collection of detailed data for each member, providing a comprehensive view of the population.

4. Policy decisions: Census inquiries are often conducted by governments to collect data for policy-making purposes. This data can be used to inform decisions related to resource allocation, social programs, infrastructure development, and more.

5. Longitudinal studies: Census inquiries can also be useful for longitudinal studies, where data is collected from the same population over a period of time. By collecting data from every member of the population, researchers can track changes and trends more accurately.

Overall, a census inquiry is a valuable method of data collection that ensures accuracy and provides a comprehensive view of the population being studied. It is particularly useful for small populations, when specific information is required, and for longitudinal studies.