Final answer:
The best-practice in AWS solution design is using multiple Availability Zones for high availability. Storing sensitive data in plaintext and using default security groups are against best practices. It's essential to encrypt sensitive data and customize security groups for enhanced security.
Step-by-step explanation:
The best-practice when designing solutions on AWS (Amazon Web Services) is to employ multiple Availability Zones (AZs) for high availability. This strategy involves deploying your applications across several physically separated locations within a region. Each Availability Zone is an isolated data center with its own power, cooling, and networking, to ensure that services are not affected by the failure of a single location.
Option 2, using a single EC2 (Elastic Compute Cloud) instance, may optimize costs in the short term, but it does not provide high availability or fault tolerance. Therefore, it's not considered a best practice for solution design that requires reliability.
Option 3, storing sensitive data in plain text, is clearly against security best practices. It's important to encrypt sensitive data to ensure its confidentiality and integrity.
Lastly, Option 4, using the default security groups, is not recommended. Best practices in cloud security encourage the creation of custom security groups that strictly follow the principle of least privilege, only allowing access that is absolutely necessary for the function of the system.