Amazon CloudWatch Events delivers a near real-time stream of system events that describe changes in Amazon Web Services (AWS) resources. Though you can generate custom application-level events and publish them to CloudWatch Events this is not the best tool for monitoring application logs.

CloudTrail is used for monitoring API activity on your account, not for monitoring application logs.

You can use CloudWatch Logs to monitor applications and systems using log data. For example, CloudWatch Logs can track the number of errors that occur in your application logs and send you a notification whenever the rate of errors exceeds a threshold you specify. This is the best tool for this requirement.

CloudWatch Metrics are the fundamental concept in CloudWatch. A metric represents a time-ordered set of data points that are published to CloudWatch. You cannot use a metric alone, it is used when setting up monitoring for any service in CloudWatch.

1. "monitor application logs and receive a notification whenever a specific number of occurrences of certain HTTP status code errors occur". This can be achieved using CW Logs only as it can monitor applications and systems using log data.

Enabling Multi-AZ isn't take place during a maintenance window. Read about the maintenance window in the third option.

Updating DB parameter groups don't take place during a maintenance window.

Periodically, Amazon RDS performs maintenance on Amazon RDS resources. Maintenance most often involves updates to the DB instance's underlying hardware, underlying operating system (OS), or database engine version. Updates to the operating system most often occur for security issues and should be done as soon as possible.

Some maintenance items require that Amazon RDS take your DB instance offline for a short time. Maintenance items that require a resource to be offline include required operating system or database patching. Required patching is automatically scheduled only for patches that are related to security and instance reliability. Such patching occurs infrequently (typically once every few months) and seldom requires more than a fraction of your maintenance window.

Promoting a Read Replica isn't take place during a maintenance window. Read about the maintenance window in the third option.

1." may force the DB to be taken offline during a maintenance window" - Security patching is the event which may force the DB to be taken offline during a maintenance window

This answer does not offer a front-end for users to upload content to.

RDS is costly compared to DynamoDB & not serverless, hence not cost-effective.

Amazon S3 alone will not provide the least latency for users around the world unless you have many buckets in different regions and a way of directing users to the closest bucket (such as Route 53 latency-based routing). However, you would then need to manage to replicate the data.

RDS is costly compared to DynamoDB and Amazon CloudFront is the lowest latency option for uploading content & this solution isn't completely serverless, hence not cost-effective.

Amazon CloudFront caches content closer to users at Edge locations around the world. This is the lowest latency option for uploading content. API Gateway and AWS Lambda are present in all options. DynamoDB can be used for storing session state data. This is a 100% serverless application.

1." cost-effective while delivering the least latency?" & "serverless application"- This means we should go for serverless, as it won't cost if you are not using it.

2. "API calls to back-end services" - API Gateway & AWS Lambda

3. "save the session state data of the user to a database" -  DynamoDB

4." accept images uploaded by users from around the world" - Amazon CloudFront for uploading, Amazon S3 for storing images.