How can the developer keep the dependencies of the Lambda functions up to date with the LEAST additional complexity?

Explanations:

Defining a maintenance window for updating Lambda functions does not directly address the issue of managing shared dependencies. It adds additional overhead for scheduling updates but does not reduce complexity or automate dependency management.

Upgrading to the most recent runtime version may improve performance or security but does not directly manage or update shared dependencies across multiple Lambda functions. This option does not solve the problem of duplicated effort in managing dependencies.

Defining a Lambda layer to contain all shared dependencies allows for centralized management of those dependencies. This reduces duplication, simplifies updates, and ensures that all Lambda functions can use the same version of the dependencies with minimal complexity.

Using an AWS CodeCommit repository to host dependencies adds complexity, as it requires additional steps for version control and dependency management. While it centralizes dependencies, it does not integrate seamlessly with Lambda functions like a Lambda layer does, making updates more cumbersome.