Sending a copy of the transaction logs to offsite storage on a daily basis would minimize the risk of losing transactions as a result of a disaster. This is because offsite storage provides a backup of the data that can be recovered in case of a catastrophic event that destroys or damages the onsite data. Storing a copy of the transaction logs onsite in a fireproof vault (B) would not protect the data from other types of disasters, such as floods, earthquakes, or theft. Encrypting or signing (D) a copy of the transaction logs and storing them on a local server would not prevent the loss of data if the server is affected by the disaster. Encryption and digital signatures are security measures that protect the confidentiality and integrity of the data, but not the availability.
Reference: CISA - Certified Information Systems Auditor Study Guide1, Chapter 5: Protection of Information Assets, Section 5.2: Backup and Recovery Concepts, Page 353.