Before march 1, 1989, the use of copyright notice was mandatory on all published works. omitting the notice on any work first published before that date could have resulted in the loss of copyright protection if corrective steps were not taken within a certain amount of time.

source: united states. copyright office. “copyright notice.” circular 3. washington, dc: library of congress, 2010. print.

this excerpt is from "copyright notice," an article found in circular 3, a publication of the united states copyright office. what is the meaning of the word "mandatory" in the sentence above?

useful
optional
required
expensive