This would not do anything for the on-premises instances. By default, your instance is enabled for basic monitoring. You can optionally enable detailed monitoring. After you enable detailed monitoring, the Amazon EC2 console displays monitoring graphs with a 1-minute period for the instance.
Basic monitoring
Data is available automatically in 5-minute periods at no charge.
Detailed monitoring
Data is available in 1-minute periods for an additional charge.

The unified CloudWatch agent can be installed on both on-premises servers and Amazon EC2 instances using multiple operating system versions. It enables you to do the following:
• Collect more system-level metrics from Amazon EC2 instances across operating systems. The metrics can include in-guest metrics, in addition to the metrics for EC2 instances.
• Collect system-level metrics from on-premises servers. These can include servers in a hybrid environment as well as servers not managed by AWS.
• Retrieve custom metrics from your applications or services using the StatsD and collectd protocols.
• Collect logs from Amazon EC2 instances and on-premises servers, running either Linux or Windows Server.

Therefore, the Development team should install the CloudWatch agent on the on-premises servers and EC2 instances. This will allow them to collect system-level metrics from servers and instances across the hybrid cloud environment.

It is incorrect as the answer does not even specify what the cron job is going to do / use for gathering and sending the data.

This is not the most efficient option as you must write and maintain custom scripts. It is better to use the CloudWatch agent as it provides all the functionality required.

1."Collect system-level metrics from on-premises servers & EC2 instances" &

2."Most efficiently"

redirects to CloudWatch agent can be installed on both on-premises servers and Amazon EC2 instances & it is most efficient also.

BatchGetItem and BatchWriteItem are used when you have multiple items to read/write. In this scenario we are updating individual items.

DeleteItem will delete single items in a table by primary key. We do not want to delete, we want to modify so UpdateItem should be used instead.

TransactWriteItems is a synchronous write operation that groups up to 25 action requests. In this scenario we are updating individual items.or Tasks.

GetItem operation returns a set of attributes for the item with the given primary key. If there is no matching item, GetItem does not return any data and there will be no Item element in the response.

PutItem creates a new item or replaces an old item with a new item. If an item that has the same primary key as the new item already exists in the specified table, the new item completely replaces the existing item.

UpdateItem edits an existing item’s attributes or adds a new item to the table if it does not already exist. You can put, delete, or add attribute values. You can also perform a conditional update on an existing item

1."Individual items" which clearly denied Batch & Transact Commands, as these are used for multiple items.

2."read, write and modify" which clearly denied Delete Command, thus remaining with only option D.

RedShift is a data warehouse for use in online analytics processing (OLAP) use cases. It is not suitable to be used as a caching layer.

Memcached does not offer read replicas or auto-failover and therefore cannot provide high availability.

DAX is used in front of DynamoDB, not Amazon RDS.

Amazon ElastiCache provides fully managed implementations of two popular in-memory data stores – Redis and Memcached. ElastiCache is a web service that makes it easy to deploy and run Memcached or Redis protocol-compliant server nodes in the cloud. The in-memory caching provided by ElastiCache can be used to significantly improve latency and throughput for many read-heavy application workloads or compute-intensive workloads. It is common to use ElastiCache as a cache in front of databases such as Amazon RDS. The two implementations, Memcached, and Redis, each offer different capabilities and limitations.Only Redis supports read replicas and auto-failover. The Redis implementation must be used if high availability is required, as is necessary for this scenario. Therefore the correct answer is to use Amazon ElastiCache Redis.

1." caching layer in front of an Amazon RDS" indicates towards ElastiCache, i.e. Option B & D,

2."maximum uptime" clearly signifies Amazon ElastiCache Redis.