Explanation:

In asymmetric key cryptography, also known as public-key cryptography, a pair of keys is used - a public key and a private key. The public key is shared with others, while the private key is kept secret.

Asymmetric key cryptography:
Asymmetric key cryptography is a cryptographic method that uses a pair of mathematically related keys for encryption and decryption. The public key is used for encryption, while the private key is used for decryption.

Public key:
The public key is available to anyone who wants to encrypt a message for the owner of the key. It is used to encrypt the message and ensure its confidentiality. The public key can be freely distributed and shared with others.

Private key:
The private key, on the other hand, is kept secret and is only known to the owner. It is used to decrypt the messages encrypted with the corresponding public key. The private key ensures the integrity and authenticity of the message.

Role of the sender and receiver:
In asymmetric key cryptography, the sender uses the recipient's public key to encrypt the message before sending it. The sender does not have access to the recipient's private key and cannot decrypt the message.

The receiver, on the other hand, uses their private key to decrypt the message received from the sender. The receiver is the only one who has access to their private key and can decrypt the message successfully.

Correct answer:
Therefore, the private key in asymmetric key cryptography is kept by the receiver. The sender does not have access to the receiver's private key and cannot decrypt the message. The private key is crucial for maintaining the security and confidentiality of the communication.