The answer you want: The U.S. Census is mandatory by federal law. There is a clear federal warning in the code about refusing to give the required information to the Census Bureau. U.S. Code, Title 13 states that citizens must comply with the census or face a $100 fine. There is a $500 penalty for giving false information.
Added fluff: I will answer in a manner that is “truthful but not helpful” to quote a well-known Democrat. My race? HUMAN. My phone number 555-1212 (information, you can reach me at that number). Etc. The number of residents will be correct because they have a right to that information, but that’s pretty close to my limit.
What’s the fine for writing illegibly (except for the words FU Obama) like I did?
Do you know if anyone has ever been charged for refusing to answer all the questions on the Census?