Which of the following encryption algorithms is applied in the PGP encryption system?

TDE (Transparent Data Encryption) is a Microsoft SQL Server and Oracle database feature used to encrypt data at rest on disk and is entirely unrelated to the PGP email and file encryption system.

Triple DES is a symmetric cipher used in many protocols such as IPsec and older SSL/TLS versions, but it was not adopted as the symmetric algorithm in the original PGP encryption standard.

Blowfish is a symmetric cipher sometimes available as an optional algorithm in later OpenPGP implementations, but it is not the algorithm that defines or is applied in the core PGP encryption system.

IDEA is a 64-bit block cipher with a 128-bit key that was selected as the symmetric encryption component in the original PGP specification created by Phil Zimmermann. It encrypts the actual message content, while RSA is used separately to protect the session key, forming PGP's hybrid encryption model.