The correct option is GitHub shows a form to enter the required inputs before the run can be started.

When a workflow is triggered with workflow dispatch and its inputs are marked as required true, the Run workflow action opens a form that displays those inputs. The run cannot begin until the user provides values for all required fields. If defaults are defined then they may appear prefilled in the form, yet the user still sees the prompt and must confirm or adjust the values before starting the run.

The workflow starts with no prompts and uses default values for inputs is incorrect because required inputs cause the interface to display a form and the run cannot start without values. Even when defaults exist, the prompt still appears and must be confirmed.

The run uses the inputs from the last manual dispatch automatically is incorrect because GitHub does not automatically reuse previous input values for a new manual run and it requires explicit entry or acceptance of defaults each time.

The workflow launches and returns an error only after a job step tries to read a missing value is incorrect because required input validation happens before the run begins and missing values block the run in the interface and through the API.

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