The correct answer is $\boxed{\text{A}}$.
The scale of a map is the ratio of a distance on the map to the corresponding distance on the ground. In this case, the scale is 1:30,000, which means that every 1 unit on the map corresponds to 30,000 units on the ground.
The focal length of a camera is the distance between the lens and the image sensor. The longer the focal length, the narrower the field of view. In this case, the focal length is 30 cm.
The flying height is the altitude of an aircraft above the ground. The flying height must be high enough to capture the entire area of interest, but not so high that the ground appears too small.
To calculate the flying height, we can use the following formula:
Flying height = (focal length * scale) / (1 + elevation)
In this case, the elevation is 1000 m, so the flying height is:
Flying height = (30 cm * 30,000) / (1 + 1000) = 4000 m
Therefore, the flying height to obtain photographs of an area of 1000 m average elevation, on scale 1:30,000, with a camera of 30 cm focal length is $\boxed{\text{4000 m}}$.
Here is a brief explanation of each option:
- Option A: 4000 m. This is the correct answer.
- Option B: 5000 m. This is too high. The flying height must be high enough to capture the entire area of interest, but not so high that the ground appears too small.
- Option C: 6000 m. This is too high. The flying height must be high enough to capture the entire area of interest, but not so high that the ground appears too small.
- Option D: 7000 m. This is too high. The flying height must be high enough to capture the entire area of interest, but not so high that the ground appears too small.