compulsory

compulsory
com‧pul‧so‧ry /kəmˈpʌlsəri/ adjective
something that is compulsory must be done because it is the law or because someone in authority orders you to ⇨ voluntary:
    the threat of compulsory redundancies
    compulsory schooling/education
    11 years of compulsory education
    Car insurance is compulsory.
—compulsorily adverb
     

THESAURUS

   ▪ compulsory if something is compulsory, you must do it because of a rule or law: Maths and English are compulsory for all students. | Compulsory education was introduced in 1870. | Wearing a seatbelt is compulsory. | It is now compulsory for anyone claiming state benefit to register with a job centre.
   ▪ obligatory [not usually before noun] if something is obligatory, you must do it because of a rule or law. Obligatory is more formal than compulsory: It is now obligatory for all competitors to wear face protectors. | Evening dress is usual, but not obligatory. | Military service is obligatory for all men between 18 and 27.
   ▪ mandatory if something is mandatory, you must do it because it is the law. Mandatory is more formal than compulsory and sounds stronger: Drug smuggling carries a mandatory death sentence. | In some countries, wearing helmets is mandatory for all cyclists.
■ not compulsory
   ▪ optional if something is optional, you can choose whether to do it or not: You don’t have to do French – it’s optional. | The holiday price includes optional excursions such as an evening cruise on the lake. | General Studies is optional for sixth-form students.
   ▪ voluntary done because you want to do it and not because you have to – used especially about things that you do because you think they will help other people or be useful: The programme is entirely voluntary, and no one will be forced to join. | The charity is funded by voluntary donations.