Which solution will meet these requirements MOST cost-effectively?

Explanations:

This option uses Amazon SQS, which is a message queuing service that is not inherently event-driven or pub/sub. SQS does not support multiple subscribers for a single message; instead, it is a point-to-point communication model. Thus, it does not meet the requirement for a publish/subscribe architecture effectively.

This option utilizes Amazon SNS, which is designed for pub/sub messaging. It allows a Lambda function to publish messages to an SNS topic, and multiple subscribers can receive these messages in real-time. This approach effectively supports an event-driven architecture and meets the requirements in a cost-effective manner since SNS has a pay-per-use pricing model.

This option involves using Amazon Kinesis Data Streams with enhanced fan-out, which is more complex and expensive than necessary for simple pub/sub messaging needs. While it supports multiple subscribers, Kinesis is generally used for real-time data processing and streaming, making it overkill for this scenario.

While this option also uses Amazon SNS for pub/sub, it combines it with an API Gateway HTTP API instead of a REST API. This is less relevant to the main requirement as both API Gateway types serve similar purposes. However, the focus on HTTP API does not provide any cost advantages over the REST API for this context and is thus less optimal than option B.