A Developer needs temporary access to resources in a second account.

What is the MOST secure way to achieve this?

A. Use the Amazon Cognito user pools to get short-lived credentials for the second account.

B. Create a dedicated IAM access key for the second account, and send it by mail.

C. Create a cross-account access role, and use sts:AssumeRole API to get short-lived credentials.

D. Establish trust, and add an SSH key for the second account to the IAM user.

Reference: https://docs.aws.amazon.com/IAM/latest/UserGuide/tutorial_cross-account-with-roles.html