Final answer:
In a scale-model atom with the nucleus the size of a tennis ball, the electron cloud would extend several kilometers away, representing the vast empty spaces within an atom.
Step-by-step explanation:
If you built a scale-model atom where the nucleus was the size of a tennis ball, then the cloud of electrons would extend several kilometers away. This is based on the fact that the distance from the atomic nucleus to the surrounding cloud of electrons is typically about 100,000 times the size of the nucleus itself. In our scaled-up world, if an atom's nucleus were 1.00 m in diameter, similar to a tennis ball, then the first electron orbit would actually begin many kilometers away, not just several centimeters or meters.
Considering the real-world scale, if an atom were scaled up to the size of a mid-sized campus, the nucleus would still be a tiny speck compared to the expanse of the campus, resembling a dust grain in a bedroom. Therefore, the correct answer to the original question would be D) several kilometers.