B & D https://aws.amazon.com/premiumsupport/knowledge-center/trusted-advisor-cost-optimization/

"you can reserve a DB instance for a one- or three-year term". We only have data for 90 days. I feel it too risky to commit for 1/3 year(s) without information on future usage. If we knew that we expected the same usage pattern for the next 1,2,3 years, Id agree with C.

B) Use the Trusted Advisor recommendations from the consolidated billing account to see all RDS instance checks at the same time. This option allows the finance team to see all RDS instance checks across all AWS accounts in one place. Since the company uses consolidated billing, this account will have access to all of the AWS accounts' Trusted Advisor recommendations. C) Review the Trusted Advisor check for Amazon RDS Reserved Instance Optimization. This check can help identify cost savings opportunities for RDS by identifying instances that can be covered by Reserved Instances. This can result in significant savings on RDS costs.

https://aws.amazon.com/premiumsupport/knowledge-center/trusted-advisor-cost-optimization/

Insights: The company runs several active high performance Amazon RDS for Oracle On-Demand DB instances for 90 days So it's clear that this company need to check the configuration of any Amazon Relational Database Service (Amazon RDS) for any database (DB) instances that appear to be idle.

B&C AWS Trusted Advisor is an online resource to help you reduce cost, increase performance, and improve security by optimizing your AWS environment. (...) Recommendations are based on the previous calendar month's hour-by-hour usage aggregated across all consolidated billing accounts. https://docs.aws.amazon.com/whitepapers/latest/cost-optimization-reservation-models/aws-trusted-advisor.html Amazon EC2 Reserved Instance Optimization: An important part of using AWS involves balancing your Reserved Instance (RI) purchase against your On-Demand Instance usage. This check provides recommendations on which RIs will help reduce the costs incurred from using On-Demand Instances. We create these recommendations by analyzing your On-Demand usage for the past 30 days. We then categorizing the usage into eligible categories for reservations. https://docs.aws.amazon.com/awssupport/latest/user/cost-optimization-checks.html#amazon-ec2-reserved-instances-optimization

If you're choosing D for the idle instances, Amazon RDS Reserved Instance Optimization Trusted Advisor check includes recommendations related to underutilized and idle RDS instances. It helps identify instances that are not fully utilized and provides recommendations on how to optimize costs, such as resizing or terminating unused instances, or purchasing reserved instances to match usage patterns more efficiently.

Reserved Instances can be shared across accounts, and that is the reason why we need to check the consolidated bill.

BC we don't want to check Idle instances because the instances were active for last 90 days. Idle means it was inactive for at least 7 days.

BD we don't want to check Idle instances because the instances were active for last 90 days. Idle means it was inactive for at least 7 days.

Reserved Instance Optimization "checks your usage of RDS and provides recommendations on purchase of Reserved Instances to help reduce costs incurred from using RDS On-Demand." In other words, it is not about optimizing reserved instances (as many here think), it about optimizing on-demand instances by converting them to reserved ones. "Idle DB Instances" check is about databases that have "not had a connection for a prolonged period of time", which we know is not the case here.

why no one considers AD. C is not the option since reserved instance is considered in case of long-term usage while it is 90 days here. But B is using consolidated billing which covers the high level billing overview of cost but not that specific for RDS running instance. should we only need to use Trust advisor for accounts where RDS is running?

The question mentions that the instances are active, so it cannot be D as it checks for idle instances

Can someone explain why so many people say it’s D and not C? It’s very clear that 90 days means reserved instances.

Answer is B & D because you can view from consolidated billing account and since RDS instances are on-demand for 90 days. There is no reserved instances. So, there is no need to check for RDS Reserved Instance Optimization.

Use the Trusted Advisor recommendations from the consolidated billing account to see all RDS instance checks at the same time and review the Trusted Advisor check for Amazon RDS Idle DB Instances

B & D is correct answer after research