6816. It is compulsory for all the students to join this tour.

✅ The correct answer is 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