Which of the following AWS services allows a database to have flexible schema and supports document data models?

The AWS service that allows a database to have flexible schema and supports document data models is Amazon DynamoDB.

Here are the steps to understand why:

1. Amazon DynamoDB is a key-value and document database that delivers single-digit millisecond performance at any scale.

2. It's a fully managed, multiregion, multimaster database with built-in security, backup and restore, and in-memory caching for internet-scale applications.

3. DynamoDB can handle more than 10 trillion requests per day and support peaks of more than 20 million requests per second.

4. One of the key features of DynamoDB is its flexible schema. This means that each row (or item, in DynamoDB terms) can have any number of columns (or attributes), and the types of values in columns can vary from row to row.

5. DynamoDB supports both key-value and document data models. In the document model, data is stored in documents made up of fields and values. Fields can vary from document to document. This flexibility makes DynamoDB a good choice for use cases that require flexible data models.