Which of the following is a symmetric cryptographic standard?

DSA (Digital Signature Algorithm) is an asymmetric algorithm that uses a public-private key pair and is designed exclusively for generating and verifying digital signatures, not for symmetric encryption.

PKI (Public Key Infrastructure) is a framework of policies, procedures, and technologies for managing asymmetric key pairs and digital certificates - it is not a cryptographic algorithm and relies on asymmetric cryptography.

RSA is an asymmetric (public-key) algorithm where a mathematically linked key pair is used - the public key encrypts or verifies, and the private key decrypts or signs - making it the opposite of symmetric cryptography.

3DES (Triple Data Encryption Standard) is a symmetric block cipher that applies the DES algorithm three times to each data block using two or three independent keys. Because the same key material is used for both encrypting and decrypting data, it is definitionally symmetric. 3DES was standardized by NIST (SP 800-67) as a stronger successor to single DES before AES became the preferred symmetric standard.