A geologist is a person who studies the earth. ... Geologists study the structure of the Earth, and they have found that it is made up of four main parts. The earth has a solid inner core, a liquid outer core, a mantle, and a crust.hence, option (D) is correct.

The correct answer is option 'D' - geophysics.

Geophysics is the scientific study of the Earth's internal structure, including its composition, physical properties, and processes. It combines principles from physics, geology, and mathematics to understand the Earth's dynamics and behavior. Geophysicists use various techniques and methods to investigate the Earth's interior, such as seismic methods, gravity measurements, magnetic field measurements, and electrical resistivity measurements.

Here is a detailed explanation of why geophysics is the study of the internal structure of the Earth:

1. Definition of Geophysics:
- Geophysics is the branch of science that deals with the physical properties and processes of the Earth and other celestial bodies.
- It involves the application of physics and mathematical principles to study the Earth's structure, composition, and dynamics.

2. Study of the Internal Structure:
- Geophysics specifically focuses on understanding the Earth's internal structure, which includes the different layers and components.
- This includes studying the Earth's crust, mantle, outer core, and inner core, as well as the boundaries between these layers.

3. Techniques Used in Geophysics:
- Geophysicists use a range of techniques to study the Earth's internal structure.
- Seismic methods involve analyzing the behavior of seismic waves generated by earthquakes or artificially induced sources. This helps determine the properties of the subsurface layers.
- Gravity measurements provide information about the distribution of mass within the Earth, which can help identify variations in density and composition.
- Magnetic field measurements help map the Earth's magnetic field, which is influenced by the distribution of magnetic minerals within the Earth's interior.
- Electrical resistivity measurements involve studying the flow of electrical currents through the Earth's subsurface layers, which can provide information about their composition and fluid content.

4. Importance of Understanding Earth's Internal Structure:
- Studying the Earth's internal structure is crucial for understanding geological processes, such as plate tectonics, volcanic activity, and earthquakes.
- It helps us understand the distribution of natural resources, such as minerals, oil, and gas.
- Geophysical studies also contribute to understanding the evolution of the Earth and its history.

In conclusion, geophysics is the study of the Earth's internal structure, involving the use of physics, geology, and mathematics to investigate its composition, physical properties, and processes.

The word "geography" is derived from the word "geo" which means "earth" , therefore study of internal structure of earth is known as geophysics.