A 150-pound person would weigh nothing at the exact center of Earth, although their mass would still be 150 pounds in everyday terms. At that point, Earth’s gravitational pulls cancel in every direction, producing zero net gravitational force and therefore zero weight.
The result follows from the spherical symmetry of a planet. Material surrounds the person on all sides, and the gravitational attraction from one direction is balanced by attraction from the opposite direction. Gravity does not simply vanish everywhere underground: it generally decreases as you move toward the center, reaching zero at the exact central point.
This answer describes weight, not mass. Mass measures how much matter a person contains and would not change. The real center of Earth is also an impossible environment for a human, with enormous pressure and temperatures of thousands of degrees. In addition, Earth is not perfectly uniform, so the idealized “nothing” answer means zero net weight in the exact center of a spherical Earth model.