The body in the solar system that is one focus of a moon's orbit is the planet that the moon orbits around.

Here's why:

* Elliptical Orbits: Moons don't orbit planets in perfect circles, but rather in elliptical paths.

* Focus Points: An ellipse has two focus points.

* Gravitational Influence: The planet's massive gravitational pull is what keeps the moon in its orbit. One focus of the elliptical orbit is located at the center of the planet.

Let me know if you'd like more information about planetary motion or orbital mechanics!