How can developers inject secrets via Vault at the time of terraform apply within their configuration file?

The correct choice is the Data Sources Block. In Terraform, the Data Sources Block is utilized to fetch information from an external source, including secrets and configuration data from HashiCorp Vault. By utilizing data sources, developers can connect to Vault and retrieve secrets dynamically at the time of the terraform apply command. This allows sensitive data to be securely injected into the Terraform configuration without hardcoding values directly into the main configuration files.

Using the Data Sources Block offers the flexibility of querying secrets that are managed and stored securely in Vault, helping to maintain best practices regarding secret management and minimizing the risk of exposure. It ensures that secrets are fetched at the moment of execution, making the process both secure and up to date with the latest values stored in Vault.

On the other hand, the options that involve the Vault Block or Secrets Block do not directly correspond to the method of retrieving secrets via a configuration file for use during apply. Similarly, a Connection Block is primarily used for setting SSH connection parameters or other similar configurations rather than for fetching secrets. Thus, the Data Sources Block is the appropriate choice for this specific use case.