Final answer:
The zenith is the direction pointing straight up from the observer's position, opposite the direction of gravity. It is therefore option 4) vertically above the observer.
Step-by-step explanation:
The zenith is defined as the point on the celestial sphere that is directly above an observer, which means it's opposite the direction of gravity for that observer. Therefore, the correct answer to the question 'The zenith defines a direction?' is option 4) vertically above the observer. The zenith is not necessarily above a point on the equator, the Sun at noon, or the North Pole unless the observer happens to be located at these positions and the conditions are correct (e.g., at noon on the equinox if on the equator).
For instance, on Earth's equator during an equinox, the Sun would indeed be at the zenith at noon. In contrast, on June 21st, the Sun would be 23° north of the celestial equator and thus 23° away from the zenith at noon for an observer located on the equator.