PKI is most secure when a certificates?

. Private keys are never exchanged, public keys are exchanged in advance of the network conversation, and revocation lists are accessible

. Private keys are exchanged in advance of a network conversation using some other secure method of transfer like a USB drive

. Decryption hash is only stored in memory on the originating host.

. Public and private keys are password protected using a certificate authority's certificate hash.

what is the correct answer?

Thanks