Which of the following is NOT a main secret engine in HashiCorp Vault?

The primary reason for identifying Excel as not being a main secret engine in HashiCorp Vault is that the secret engines in Vault are specifically designed to manage various types of secrets, such as sensitive data, credentials, and certificates. The Key/Value engine is used for storing arbitrary secrets in key-value pairs, while the Database engine facilitates dynamic generation and management of database credentials. The PKI (Public Key Infrastructure) engine is responsible for generating and managing certificates.

Excel, on the other hand, is a spreadsheet application and does not serve the purpose of secret management within the context of Vault. It does not have functionalities aligned with the needs for securing, managing, and controlling access to sensitive data. The main secret engines in HashiCorp Vault are focused on providing robust, security-oriented solutions that meet specific use cases relevant to organizations managing sensitive information.