Final answer:
The symmetric key in cryptography must be a secret passphrase to ensure security, as it is used for both encryption and decryption of data.
Step-by-step explanation:
The question pertains to the nature of a symmetric key used in cryptographic systems. In symmetric key cryptography, the same key is used for both encryption and decryption of data. For effective security, the symmetric key must be kept secret between the parties involved. The correct answer to the question is that the symmetric key must be a secret passphrase. A random number could be part of the process to generate such a passphrase, but it's not the key itself. Prime numbers are more often associated with asymmetric cryptography, not symmetric. Lastly, a public key is part of asymmetric cryptography where different keys are used for encryption and decryption.