Amazon Simple Notification Service (Amazon SNS) is the correct choice because it provides a publish subscribe model with push delivery and it can fan out messages to large numbers of subscribers across email and mobile apps at the same time.

SNS supports multiple protocols such as email, SMS, mobile push and HTTP and HTTPS and it is built to scale for immediate delivery to thousands of subscribers without requiring consumers to poll. That push and fan out behavior matches the requirement to alert about 25,000 subscribers simultaneously.

Amazon Simple Queue Service (Amazon SQS) uses a pull model where consumers poll queues and it is intended for decoupling and buffering rather than immediate push fan out to many recipients.

Amazon EventBridge routes events between services using an event bus and it is designed for application integration and routing rather than direct end user push notifications.

AWS Step Functions coordinates workflows and state machines and it is not a notification delivery service for broadcasting alerts to subscribers.

Full AWS Practitioner Certification Question