The correct answer is: A. 1 millimeter.
The water vapor in the atmosphere is very thin. If all of it were to condense and fall to the ground, it would only cover the globe with a layer of water about 1 millimeter thick. This is equivalent to about 0.04 inches.
The other options are incorrect because they are much thicker than the actual amount of water vapor in the atmosphere. For example, 1 inch of water would cover the globe with a layer of water that is 25.4 millimeters thick. This is about 625 times thicker than the actual amount of water vapor in the atmosphere.
It is important to note that the amount of water vapor in the atmosphere varies depending on the time of year and the location. For example, there is more water vapor in the atmosphere during the summer than during the winter. There is also more water vapor in the atmosphere near the equator than near the poles.