>>6855120 (lb)

According to United States Code, Title 13 (Census), Chapter 7 (Offenses and Penalties), SubChapter II, if you're over 18 and refuse to answer all or part of the Census, you can be fined up to $100. If you give false answers, you're subject to a fine of up to $500.

It should be noted…nobody has ever been fined for refusing to answer questions.