A user issues the following cURL command to encrypt data using the transit engine and...

The payload.json file that has the correct contents is C. This file contains a JSON object with a single key, “plaintext”, and a value that is the base64-encoded string of the data to be encrypted. This is the format that the Vault API expects for the transit encrypt endpoint1. The other files are not correct because they either have the wrong key name, the wrong value format, or the wrong JSON syntax.

References:

Encrypt Data - Transit Secrets Engine | Vault | HashiCorp Developer