Which of the following is an example of an asymmetric encryption implementation?

SHA1 is a cryptographic hash function, not an encryption algorithm - it produces a one-way fixed-length digest and does not use keys for encryption or decryption.

PGP (Pretty Good Privacy) is an asymmetric encryption system that uses a public key to encrypt data and a corresponding private key to decrypt it, along with digital signatures for authentication. It implements the RSA or DSA algorithms for key exchange and signing, making it a direct example of asymmetric cryptography used in real-world email and file encryption.

3DES (Triple DES) is a symmetric encryption algorithm that uses the same shared secret key for both encryption and decryption, making it the opposite of asymmetric.

MD5 is a cryptographic hash function that generates a one-way message digest and is not an encryption algorithm at all - it uses no key and cannot be reversed to recover original data.