Which legal concept signifies a failure to act when there is a duty to do so?

The correct answer is "omission." This term specifically refers to a situation where an individual fails to take action when there is a legal duty to act. In criminal law, a person may be held liable for a crime if their inaction leads to harm, particularly if they had a responsibility to intervene or provide assistance. An example of this could be a caregiver failing to provide necessary medical attention to someone in their care, resulting in harm.

The other choices refer to different legal concepts: "mens rea" pertains to the mental state or intent behind committing a crime, "actus reus" refers to the physical act or conduct that constitutes a criminal offense, and "negligence" generally relates to a failure to exercise the care that a reasonably prudent person would exercise in similar circumstances, which can involve acts of omission but is broader in scope. The specificity of "omission" makes it the most accurate term for the failure to act under a legal obligation.