The correct answer is $\boxed{\text{C}}$.
The relief displacement of a building is the horizontal distance between the top of the building and the line of sight from the camera to the top of the building. The flying height of the camera is the distance between the camera and the mean ground level. The distance of the top of the building from a nadir point is the horizontal distance between the top of the building and the camera.
The height of the building can be calculated using the following formula:
$h = \frac{d}{f} \times H$
where:
$h$ is the height of the building
$d$ is the relief displacement of the building
$f$ is the focal length of the camera
$H$ is the flying height of the camera
In this case, $d = 7.2 \text{ mm}$, $f = 100 \text{ mm}$, and $H = 1000 \text{ m}$. Substituting these values into the formula, we get:
$h = \frac{7.2 \text{ mm}}{100 \text{ mm}} \times 1000 \text{ m} = 72 \text{ m}$
Therefore, the height of the building is $\boxed{72 \text{ m}}$.
Option A is incorrect because it is the height of the building above the mean ground level, not the height of the building above the nadir point. Option B is incorrect because it is the distance of the top of the building from the nadir point, not the height of the building. Option D is incorrect because it is the flying height of the camera, not the height of the building.