Answer: AES256

The Advanced Encryption Standard (AES) with a 256-bit key is the encryption algorithm used for encrypting data at the infrastructure level in Google Cloud. While Blowfish and RSA are robust encryption algorithms, they are not employed at the infrastructure level of GCP. The Data Encryption Standard (DES), which uses a 56-bit key, is deemed insufficient for securing data due to its vulnerability. For more details, refer to [Google Cloud's encryption documentation](https://cloud.google.com/security/encryption/default-encryption).

Author: LeetQuiz Editorial Team