Which of the following tools can be used to enable authentication for a large number of users accessing an Amazon S3 bucket using a mobile app?

Answer options:

A.AWS IAM user
B.Amazon Cognito
C.AWS IAM group
D.AWS STS

Correct Answer: B
Using Amazon Cognito user pool & identity pool, mobile app users can be authenticated to use AWS services like Amazon S3 bucket.
Options A & C are incorrect as IAM users & IAM group can create authentication & manage access for users accessing AWS services. These services are not suitable to provide authentication to millions of users accessing AWS services via mobile app.
Option D is incorrect as AWS STS is a service that grants temporary access privilege for AWS IAM users or federated users.
For more information on Amazon Cognito, refer to the following URL:
https://aws.amazon.com/cognito/faqs/