Step 1: Quality control ranks defects on a severity scale: critical > major > minor.
Step 2: Critical defects stop the product working or create hazard. Major defects noticeably reduce usability. Minor defects are the mildest class, the item still serves its purpose without a real drop in usability.
Step 3: Checking the choices: stopping the process function (A) fits critical/major, not minor. Being fixable during production (C) or preventable beforehand (D) describe handling of the defect, not its severity, so neither defines "minor".
Step 4: The statement matching "mild severity, usability barely affected" is the one where the defect does not significantly impact performance or usability.
\[\boxed{\text{Minor defect: does not significantly impact performance or usability}}\]