Through which of the following has Codex functionality been made available?

This question has more than one right answer, and that is the substance of it: the same underlying agent is exposed through several front doors, because people work in different places.

The terminal client suits anyone who lives in a shell and wants the tool scriptable — it runs interactively or gets called from a pipeline. The editor extension puts it beside your code, so open files and the lines you have selected become context without your restating them. There is programmatic access for building it into something else, and a cloud version that runs a task in its own environment and can be kicked off from the web or from tools like GitHub, GitLab, Linear and Slack.

These are entry points onto one capability, not four different products, so choosing between them is a question about where your work already happens. The one wrong answer is the only option that is not a way of running anything — and its word "only" would rule out the three surfaces that genuinely exist.