The circumstances that allow a person to view the curve of the Earth are:
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Viewing the horizon from a plane that is flying above 35,000 feet: At high altitudes, such as in a commercial airplane, the curvature of the Earth becomes more visible due to the broader perspective of the horizon.
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Seeing the shadow of the Earth as a curve on the Moon during a lunar eclipse: During a lunar eclipse, the Earth's shadow falls on the moon, and this shadow is always circular due to the round shape of the Earth, which is another indication of the Earth's curvature.
The other two options you mentioned do not provide a direct view of the Earth's curvature:
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Watching a sailboat sail into or from a harbor and crossing the horizon: This phenomenon can happen due to perspective and atmospheric conditions, but it doesn't provide a direct view of the Earth's curvature.
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Dropping balls from two buildings, and seeing the balls fall and converge to a point far below the surface: This is more related to gravity and perspective rather than providing a view of the Earth's curvature.
So, the first and third options are the most indicative of observing the curvature of the Earth.