A challenge to Public Key Cryptography Systems that is not a challenge to Secret Key Cryptography Systems is:

a$.$ the need of an individual user to keep track of a key for each distinct party with whom the user communicates.

b$.$ the need to protect keys from being guessed or stolen.

c$.$ the need to encrypt messages using a key when securing communications.

d$.$ the need to establish trusted authorities that provide computer services to distribute certain keys electronically.