Which of the following tools for application portability and development environments is an open source software application that creates and configures virtual development environments?

The correct choice is Vagrant, as it is specifically designed to create and configure virtual development environments in a portable manner. Vagrant allows developers to define their runtime environments using a simple configuration file, enabling them to replicate environments consistently across different systems. It supports various virtualization platforms by creating virtual machines that are lightweight and portable, making it highly effective for development and testing.

VirtualBox, while it is a popular virtualization tool, primarily functions as a hypervisor to run virtual machines and does not inherently manage the configuration or portability of development environments like Vagrant does. Packer is another open-source tool used for creating machine images, but it focuses more on building images for various platforms rather than configuring development environments per se. Docker, although it plays a crucial role in containerization and also helps with creating consistent development environments, operates differently by leveraging containers rather than full virtual machines like Vagrant, which is tailored for that specific purpose.

Overall, Vagrant's role in defining, provisioning, and managing development environments makes it the ideal choice for the question posed.