Answer:
Option D, The different layers of Earth exert pressure to keep the inner core solid.
Step-by-step explanation:
Due to extreme high pressure at the core of the earth, the iron does not convert into liquid form. According to a research conducted by KTH Royal Institute of Technology, the iron acquires the body-centred cubic structure when it is at normal pressure but when the pressure is extremely high the iron acquires the shape of hexagonal with 12 points which is a close packed structure. Due to this close pack structure, iron remains solid in the inner core of earth.
Hence, option D is correct