In the structure of planet Earth, below the mantle, the core is mainly made up of Option 3: Iron.

The Earth`s core is divided into two parts: the outer core and the inner core. The outer core, which lies between the mantle and the inner core, is primarily composed of liquid iron alloyed with smaller amounts of nickel and other elements. The inner core, on the other hand, is believed to be a solid ball consisting mainly of iron.

There are several lines of evidence that support the understanding of iron being the primary constituent of the Earth`s core:

1. Seismic Waves: One of the most significant pieces of evidence comes from the study of seismic waves generated by earthquakes. Seismic waves can travel through the Earth and provide valuable information about its interior structure. The behavior of these waves suggests that the core must be composed of heavy elements, with iron being the most likely candidate due to its abundance and high density.

2. Meteorites: Another source of evidence comes from the study of meteorites, which are remnants of primitive materials from the early Solar System. Some meteorites, known as iron meteorites, are composed almost entirely of iron-nickel alloys. It is believed that these meteorites originated from the cores of smaller planetary bodies that formed during the early stages of the Solar System`s formation. This similarity in composition between iron meteorites and Earth`s core provides further support for the presence of iron in the Earth`s core.

3. Laboratory Experiments: Scientists have conducted high-pressure and high-temperature experiments in laboratories to simulate the conditions present in the Earth`s core. These experiments have confirmed that iron, under such extreme conditions, exhibits the properties necessary to explain the observed behavior of seismic waves. These findings further strengthen the understanding that iron is the main constituent of the Earth`s core.

It is worth noting that while iron is the primary element in the Earth`s core, it is not the only element present. Other elements, such as nickel and smaller amounts of lighter elements, may also be present. However, iron is the dominant component, accounting for a significant portion of the core`s composition.