Each month, the census bureau mails survey forms to 250,000 households asking questions about the people living in the household and about such things as motor vehicles and housing costs. telephone calls are made to households that do not return the form. in one month, responses were obtained from 240,000 of the households contacted. the population of interest is:

The 250,000 households that were initially contacted by mails

Explanation:

The population is the totality of either animate or inanimate objects that forms the target of a research, the main target of the researchers inquiry is the population, in fact the population is the target beneficiary of the exercise.  

In this question the main target is the 250,000 households that the census bureau mailed, these are the people the bureau had in mind and they form the population of interest not the actual responses obtained.

a is the correct answer, tell me if im right