Which of the following tools is primarily used for configuration management and written in Ruby?

Chef is a powerful tool specifically designed for configuration management, and it is primarily written in the Ruby programming language. Configuration management involves maintaining computer systems, servers, and software in a desired, consistent state, and Chef facilitates this by enabling automation of setup, deployment, and configuration tasks across infrastructure.

The design of Chef is predicated on a client-server model, where configuration scripts, referred to as "recipes," are written in Ruby and define how systems should be configured. This allows for flexibility and extensibility, leveraging Ruby's capabilities for complex configurations and data manipulation.

Other tools mentioned, such as Ansible, Puppet, and SaltStack, serve similar functions within the domain of automation and configuration management but are based on different technologies. Ansible is known for its simplicity and is written in Python, Puppet utilizes its own domain-specific language but is primarily based on Ruby as well, while SaltStack combines a lightweight agent with a centralized management approach, again not primarily relying on Ruby. Thus, Chef remains distinct for its Ruby-centric framework tailored for configuration management tasks.