The fundamental difference between symmetric and asymmetric key cryptographic systems is that symmetric key cryptography uses which of the following?

Using multiple keys for non-repudiation describes asymmetric cryptography, where a private key is used to sign data, not symmetric cryptography which uses a single shared key.

Using different keys on both ends of the transport medium describes asymmetric cryptography, where one party holds a public key and the other holds a private key.

The transmission medium type such as fiber is irrelevant to the definition of symmetric key cryptography, and symmetric encryption is not limited to fiber or to bulk transmission by definition.

Symmetric key cryptography requires that both the sender and receiver possess and use the identical secret key for both encryption and decryption. This shared secret must be securely distributed to all parties in advance, which is the defining characteristic that differentiates it from asymmetric systems.