Final answer:
To allow customers outside of your VPC to use Kibana with Elasticsearch, set up an Amazon Cognito authentication for Kibana.
Step-by-step explanation:
The correct answer is C) Set up an Amazon Cognito authentication for Kibana. To allow customers outside of your VPC to use Kibana with Elasticsearch, you can set up an Amazon Cognito authentication. Amazon Cognito provides a secure way to authenticate and authorize access to your Kibana instance. By setting up Amazon Cognito, you can create an authentication system that allows customers to access Kibana securely without sharing your VPC credentials.