Final answer:
Calvin should be charged with carjacking, as he took a motor vehicle from Helen through the threat of force using a knife. This crime fulfills the requirements of carjacking in law.
Step-by-step explanation:
It would be most appropriate to charge Calvin with carjacking. The crime of carjacking is defined as taking a motor vehicle from the immediate presence of a person by force or threat of force. In this scenario, Calvin took a motor vehicle from Helen by threatening her with a knife, which fulfills the requirements of carjacking. Moreover, considering Helen's age, some jurisdictions might impose a harsher penalty as some places have specific provisions for crimes committed against elderly persons, being 62 years of age may classify her within that category.