Final answer:
The crime of snatching a purse containing $399 from the victim's hand while brandishing a knife at the victim is robbery.
Step-by-step explanation:
The crime of snatching a purse containing $399 from the victim's hand while brandishing a knife at the victim is robbery (option a).
Robbery is defined as taking someone's property through the use of force or threat of force. In this case, the perpetrator snatched the purse from the victim's hand while brandishing a knife, which constitutes force and intimidation. The fact that the purse contained $399 is not directly relevant to the classification of the crime.
It is important to note that each jurisdiction may have slightly different legal definitions, but in general, this would be considered a case of robbery.