A developer needs temporary access to resources in a different AWS account. Which approach provides the most secure temporary access?

Creating a cross-account IAM role in the target account and using sts:AssumeRole is the most secure and recommended method for temporary cross-account access. This approach grants temporary security credentials with a defined expiration, eliminating the need to share long-term credentials. The calling account assumes the role, inheriting its permissions for a limited duration. Using Amazon Cognito user pools is for user authentication and authorization within an application, not for granting programmatic cross-account AWS resource access. Creating and mailing IAM access keys is highly insecure as it involves sharing long-term credentials over an unencrypted channel, posing a significant security risk. Establishing trust and adding an SSH key to an IAM user is for secure shell access to EC2 instances, not for programmatic cross-account AWS API access.