What does remiss mean?

The term "remiss" refers to a state of being careless or neglectful, particularly in regard to one's duties or responsibilities. When someone is described as remiss, it implies that they have failed to take proper action or demonstrate the necessary attention or diligence required in a situation. This definition aligns well with the meaning of being neglectful in the performance of one's duty, making it the most accurate choice.

In contrast, the other options present different meanings unrelated to the concept of negligence. For instance, describing something as unconquerable or refusing to yield pertains to a sense of strength or durability, while sincerity and authenticity relate to the genuineness of emotions or actions. Lastly, being extremely poisonous and full of malice indicates a nature that is dangerous and malicious, which is distinctly opposite to the careless disposition implied by "remiss."