Synonym of OBLIGATORY

Answer: Option B

Useful : able to be used for a practical purpose or in several ways.
Required : officially compulsory, or otherwise considered essential; indispensable.
Stubborn : having or showing dogged determination not to change one's attitude or position on something, especially in spite of good arguments or reasons to do so.
Agreeable : quite enjoyable and pleasurable; pleasant.
Obligatory : required by a legal, moral, or other rule; compulsory.

Synonym of Obligatory is Required