The correct answer is (c), motive.
Actus reus is the physical element of a crime, while mens rea is the mental element. Motive is not a necessary element of a crime, although it may be considered by a court in determining the appropriate sentence.
For example, if a person shoots and kills another person, the actus reus of the crime would be the act of shooting the person. The mens rea of the crime would be the intent to kill the person. The motive for the crime might be revenge, jealousy, or hatred. However, the motive is not necessary to establish that the person committed the crime.
In some cases, motive may be considered by a court in determining the appropriate sentence. For example, if a person kills another person in cold blood, the sentence may be more severe than if the person kills another person in self-defense. However, motive is not a necessary element of a crime.