I agree with the suggested answer B.
Reasoning: The most efficient and secure way to share encrypted RDS snapshots between AWS accounts is by modifying the KMS key policy to grant access to the other accounts and then sharing the snapshot. This allows the other accounts to create their own RDS instances or EBS volumes from the snapshot while maintaining the encryption at rest.
Sharing encrypted RDS snapshots requires granting permissions to the other accounts to use the KMS key used to encrypt the snapshot. Without the correct permissions, the other accounts will not be able to restore the snapshot.
This method is secure and compliant with the requirement that all data must be encrypted at rest.
Reasons for not choosing other options:

Option A involves decrypting the snapshot, which is less secure than sharing the encrypted snapshot directly.
Option C involves creating an EC2 instance, which is an unnecessary step and more complex than directly sharing the snapshot.
Option D involves creating an unencrypted RDS instance and exporting the data, which violates the requirement that all data must be encrypted at rest. It is also a complicated and insecure process.

The correct answer is B: Update the key policy to grant permission to the AWS KMS encryption key used to encrypt the snapshot with all relevant accounts, then share the snapshot with those accounts.

Detailed Explanation:
The most secure and direct method to share an encrypted RDS snapshot across different AWS accounts involves these steps:

Modify the KMS Key Policy: Add the AWS account IDs of the accounts you want to share the snapshot with to the KMS key policy. This grants these accounts permission to use the KMS key.
Share the Encrypted Snapshot: Share the encrypted RDS snapshot with the AWS accounts you granted permission to in the KMS key policy.

This approach ensures that the data remains encrypted at rest throughout the sharing process and leverages AWS's native encryption and sharing capabilities.

Here's why the other options are not suitable:

A. This option involves decrypting the snapshot, which is not ideal because it exposes the data in an unencrypted state. It is less secure than sharing the encrypted snapshot directly.
C. Creating an EC2 instance and then creating a snapshot from its EBS volume adds unnecessary complexity. It does not directly address the requirement of sharing an RDS snapshot.
D. Creating an unencrypted RDS instance and exporting the data completely violates the requirement that all data must be encrypted at rest. This method is highly insecure.

Therefore, the correct answer is B.

The key to this question lies in understanding how AWS KMS keys and policies work in conjunction with sharing encrypted resources. By granting the necessary permissions in the KMS key policy, the other accounts can use their own keys to decrypt the RDS snapshots.

In summary, option B aligns with security best practices and the specific requirements of the scenario by utilizing KMS key policies to securely share the encrypted snapshot between accounts.

By granting permissions in the KMS key policy, the other accounts can then restore the RDS snapshot and use the data, while keeping the data secure and encrypted.

Therefore, the correct answer is B.

The most efficient and secure way to share encrypted RDS snapshots between AWS accounts is by modifying the KMS key policy to grant access to the other accounts and then sharing the snapshot. This allows the other accounts to create their own RDS instances or EBS volumes from the snapshot while maintaining the encryption at rest.

The correct answer is B: Update the key policy to grant permission to the AWS KMS encryption key used to encrypt the snapshot with all relevant accounts, then share the snapshot with those accounts.

In addition to the above reasons, here are some additional considerations that support option B:

Simplicity: Option B is the simplest and most straightforward approach. It does not involve any unnecessary steps or complexities.
Scalability: Option B is scalable and can be easily adapted to share snapshots with multiple accounts.
Maintainability: Option B is easy to maintain and manage.

Final Answer: The final answer is B: Update the key policy to grant permission to the AWS KMS encryption key used to encrypt the snapshot with all relevant accounts, then share the snapshot with those accounts.

Because this approach satisfies the requirements of sharing encrypted RDS snapshots between accounts in a secure and efficient manner, and aligns with the best practices for using KMS keys and policies.