Which of the following tools is not part of the Chef ecosystem?

In the context of the Chef ecosystem, the correct choice refers to Git, which is a version control system rather than a tool specifically developed for or part of Chef. Test Kitchen, ChefSpec, and Cookstyle are all integrated tools designed to work within the Chef automation framework.

Test Kitchen is used for testing and developing Chef cookbooks; it allows developers to spin up environments to test their configurations in a cloud or local setup. ChefSpec is a testing framework that allows developers to write RSpec tests for their Chef cookbooks. Cookstyle is a code linter and formatter for Chef code that ensures adherence to best practices and style guidelines.

While Git is essential for version control and collaboration in software development, it is not specifically designed for or integrated into the Chef ecosystem in the same way as the other mentioned tools. Therefore, identifying Git as a tool not part of the Chef ecosystem is accurate given the context of the other options.