Answer:
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.