Correct Answer & Explanation
Option C
• The verb 'know' is a stative verb, meaning it describes a state or condition rather than an action, and is generally not used in continuous tenses.
• The phrase 'for the last twenty years' indicates a duration that started in the past and continues up to the present.
• For stative verbs with a time expression indicating duration up to the present, the Present Perfect Tense (have/has + V3) is used.
• Therefore, 'have known' is the correct form.