It is compulsory for all the students to join this tour.

Answer: Option D

Regular : arranged in or constituting a constant or definite pattern, especially with the same space between individual instances.
Necessary : needed to be done, achieved, or present; essential.
Dutiful : conscientiously or obediently fulfilling one's duty.
Obligatory : required by a legal, moral, or other rule; compulsory.
Compulsory : required by law or a rule; obligatory.

Synonym of Compulsory is Obligatory