Thickness of Earth’s core is approximately:

The Earth's core is the innermost layer of our planet, composed primarily of iron and nickel. It is divided into two parts: the liquid outer core and the solid inner core. The outer core begins at a depth of approximately 2,900 km below the Earth's surface, extending down to about 5,150 km. The inner core then extends from 5,150 km down to the very center of the Earth, which is at roughly 6,371 km depth. Therefore, the total thickness of the Earth's core, from the core-mantle boundary to the center, is approximately 6,371 km - 2,900 km = 3,471 km. Among the given options, 3900 km is the closest approximation to this total thickness, accounting for both the outer and inner core components.

• 500 km is far too small to represent the total thickness of the Earth's core. This dimension is more comparable to the thickness of the Earth's crust or a shallow part of the mantle.
• 2900 km represents the approximate depth at which the core begins (the core-mantle boundary), not its total thickness.
• 1900 km is also significantly too small to be the total thickness of the Earth's core. It represents less than half of the actual core's depth.