Explanation: The IAM Documentation states the following
• IAM managed CM Ks: You cannot manage key rotation for IAM managed CMKs. IAM KMS automatically rotates IAM managed keys every three years (1095 days).
Note: IAM-managed CMKs are rotated every 3yrs, Customer-Managed CMKs are rotated every 365-days from when rotation is enabled.
Option A, B, C are invalid because the dettings for automatic key rotation is not changeable.
For more information on key rotation please visit the below URL
https://docs.IAM.amazon.com/kms/latest/developereuide/rotate-keys.html
IAM managed CMKs are CMKs in your account that are created, managed, and used on your behalf by an IAM service that is integrated with IAM KMS. This CMK is unique to your IAM account and region. Only the service that created the IAM managed CMK can use it
You can login to you IAM dashbaord . Click on "Encryption Keys"
You will find the list based on the services you are using as follows:
• IAM/elasticfilesystem 1 IAM/lightsail
• IAM/s3
• IAM/rds and many more
Detailed Guide: KMS
You can recognize IAM managed CMKs because their aliases have the format IAM/service-name, such as IAM/redshift. Typically, a service creates its IAM managed CMK in your account when you set up the service or the first time you use the CMfC
The IAM services that integrate with IAM KMS can use it in many different ways. Some services create IAM managed CMKs in your account. Other services require that you specify a customer managed CMK that you have created. And, others support both types of CMKs to allow you the ease of an IAM managed CMK or the control of a customer-managed CMK
Rotation period for CMKs is as follows:
• IAM managed CMKs: 1095 days
• Customer managed CMKs: 365 days
Since question mentions about "CMK where backing keys is managed by IAM", its Amazon(IAM) managed and its rotation period turns out to be 1095 days{every 3 years)
For more details, please check below IAM Docs:
https://docs.IAM.amazon.com/kms/latest/developerguide/concepts.html
The correct answer is: After 3 years
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