When it comes to fulfilling a duty or responsibility, there are plenty of synonyms for the phrase "are one's obligation." Some common options include "are one's duty," "are one's responsibility," "are required," "are expected," "are necessary," "are incumbent upon," and "are compulsory." Depending on the context and tone of the situation, you may choose to use one of these alternatives in place of "are one's obligation." For example, if you want to convey a sense of urgency or importance, you might use "are necessary" or "are incumbent upon," while if you want to emphasize that the action is simply part of one's role or job, you might use "are one's duty" or "are one's responsibility".
