The correct option is
D obligatory
In this sentence we would be looking into am alternate for the word "compulsory" from the options given below,
Compulsory means required by law or a rule; obligatory.
Obligatory means required by a legal, moral, or other rule; compulsory. It is the correct option.
If we look into the other options,
Regular means arranged in or constituting a constant or definite pattern, especially with the same space between individual instances
Necessary means needed to be done, achieved, or present; essential.
Dutiful means conscientiously or obediently fulfilling one's duty
Option D is correct