'Actus non facit reum nisi mens sit rea' is a fundamental maxim of criminal law. It translates to "an act does not make a man guilty unless his mind is also guilty." This means that to constitute a crime, there must be a combination of two elements:
Actus Reus: The guilty act or physical element of the crime.
Mens Rea: The guilty mind or mental element of the crime (e.g., intention, knowledge, recklessness).
Option (a) is the most accurate and complete translation of this principle. Options (b) and (c) are also correct in substance but (a) is the classic expression.