She ___ him for five years:

The correct answer is "has known". This uses the Present Perfect tense, which is appropriate for actions that started in the past and continue up to the present moment. The phrase "for five years" clearly indicates a duration that began in the past and extends to now.

• "know" and "knows" are Simple Present, used for habitual actions or facts, not for actions with ongoing duration from the past.
• "is knowing" is Present Continuous, which is generally not used with stative verbs like "know" to describe a continuous state, especially not with a specified duration.