The `plain_text_input` element is specifically intended to collect free-form textual information from users. Slack allows the element to accept either single-line or multiline text and commonly places it inside an `input` block when constructing a modal form. The application receives the entered value when the modal generates a `view_submission` payload, allowing the backend to validate and process the submitted information. A button initiates an action rather than collecting text. A `static_select` element presents predefined choices, while an overflow menu provides a compact collection of predefined actions or selections. Neither alternative allows unrestricted textual entry. Slack's Block Kit element reference explicitly identifies the plain-text input element as the component for entering free-form data. Therefore, B is the correct element for collecting textual input inside a modal.