Choice "D" is correct. With symmetric encryption, both parties use the same key to encrypt and decrypt the message so that the key must be shared. This would require a unique private key for each entity with which one wanted to share encrypted data. In asymmetric encryption, the private key is not shared and the public key provides the other half necessary to encrypt/decrypt.Choice "a" is incorrect. The private key in symmetric encryption has to be transmitted to the receiver in some manner. Breaking the private key into fragments is a way to attempt to keep the private key as secure as possible.
Choice "c" is incorrect. In symmetric encryption, both the sender and the receiver must have the private key.
Choice "b" is incorrect. In symmetric encryption, both the sender and the receiver must have the private key.