Public key cryptography is

1) a cryptography system that is widely known and published.

2) a cryptography system that depends on the recipient of a message having a pair of keys, one of which is widely available.

3) a cryptography system that depends on the sender of a message having a pair of keys, one of which is widely available.

4) a cryptography system that depends on sender and recipient of a message each having a copy of the same key.