Question

What body in the solar system do you think is one focus of the moons orbit

Answer 1

-- From the point of view of an observer on Earth, the moon's orbit is an ellipse
with the Earth's center at one focus.

-- From the point of view of an observer on the sun, or on any other planet,
the moon's orbit is an ellipse with the sun at one focus, and perturbed by being
near the Earth.

Answer 2

The Earth.

Orbits are elliptical in shape with the body being orbited at one focus. So the Sun is at one focus of the Earth's orbit around it.