* Object Store and VM Store is used for sharing data inter or intra mule applications in same setup. Can't be used with external Business Partner
* Also File connector will not be useful as the two companies currently have no shared IT infrastructure. It's specific for local use.
* Correct answer is SFTP connector. The SFTP Connector implements a secure file transport channel so that your Mule application can exchange files with external resources. SFTP uses the SSH security protocol to transfer messages. You can implement the SFTP endpoint as an inbound endpoint with a one-way exchange pattern, or as an outbound endpoint configured for either a one-way or request-response exchange pattern.

We need to note few things about the scenario which will help us in reaching the correct solution.
Point 1 : The APIs are all publicly available and are associated with several mobile applications and web applications. This means Apply an IP blacklist policy is not viable option. as blacklisting IPs is limited to partial web traffic. It can't be useful for traffic from mobile application Point 2 : The organization does NOT want to use any authentication or compliance policies for these APIs. This means we can not apply HTTPS mutual authentication scheme.
Header injection or removal will not help the purpose.
By its nature, JSON is vulnerable to JavaScript injection. When you parse the JSON object, the malicious code inflicts its damages. An inordinate increase in the size and depth of the JSON payload can indicate injection. Applying the JSON threat protection policy can limit the size of your JSON payload and thwart recursive additions to the JSON hierarchy.
Hence correct answer is Apply a JSON threat protection policy to all APIs to detect potential threat vectors

Explanation
https://www.folkstalk.com/2019/11/mulesoft-integration-and-platform.html