The word “obligatory” means “required by a legal, moral, or other rule; compulsory.” For example: "use of seat belts in cars is now obligatory"
Option B: “Required” means “officially compulsory, or otherwise considered essential; indispensable.” For example: "eight editions were published, each required reading for trainees" This is the closest to the meaning of the word “obligatory”. Hence, this is the correct answer.
Option A: “Useful” means “able to be used for a practical purpose or in several ways.” For example: "aspirins are useful for headaches" This is a completely different meaning. Hence, this option is incorrect.
Option C: “Stubborn” means “having or showing dogged determination not to change one's attitude or position on something, especially in spite of good reasons to do so.” For example: "you're a silly, stubborn old woman" This is a completely different meaning. Hence, this option is incorrect.
Option D: “Agreeable” means “willing to agree to something.” For example: "they were agreeable to its publication" This is a completely different meaning. Hence, this option is incorrect.