The Earth assumes the shape of an oblate ellipsoid because of its rotation. Option 1 correctly states that the Earth`s rotation causes it to bulge slightly at the equator and flatten at the poles.

When the Earth rotates on its axis, the centrifugal force generated by this rotation pushes outward, causing the equatorial region to bulge. This bulging results in a larger radius at the equator compared to the polar regions, leading to the flattened shape at the poles.

Option 2, which states that the Earth bulges at the poles and flattens at the equator due to rotation, is incorrect. The opposite is true; the bulging occurs at the equator.

Option 3, which suggests that the Earth`s revolution (orbiting the Sun) is responsible for the oblate shape, is also incorrect. While revolution affects the Earth`s orbit and the seasons, it does not directly influence the Earth`s shape.

Option 4 is also incorrect, as it suggests that the Earth bulges at the poles and flattens at the equator due to revolution, which is not the case.

Overall, the Earth`s rotation is the primary factor that causes it to assume the shape of an oblate ellipsoid.