The correct answer is option 2, an oblate spheroid.
An oblate spheroid is a three-dimensional geometric shape that is formed by spinning an ellipse around its shorter axis. It is characterized by a slightly flattened shape at the poles and a bulging in the middle, similar to a sphere that has been squashed slightly.
Option 1, a sphere, is not the correct answer because while the earth is nearly round, it is not a perfect sphere. It is slightly flattened at the poles and bulges at the equator.
Option 3, a prolate spheroid, is not correct because it is the opposite of an oblate spheroid. A prolate spheroid is formed by spinning an ellipse around its longer axis, resulting in a shape that is elongated along its axis.
Option 4, a tetrahedron, is not correct because a tetrahedron is a four-sided polyhedron with triangular faces, which does not accurately represent the shape of the earth.
Therefore, the correct answer is option 2, an oblate spheroid.