The melting point of a substance is the temperature at which it changes from a solid state to a liquid state at atmospheric pressure. It is an important physical property of a substance and can provide valuable information about its characteristics and behavior.

The correct answer to the question is option 'D', which states that both the solid and liquid phases of the substance exist together at the melting point. This means that during the process of melting, as the temperature increases, both the solid and liquid phases coexist until the solid is completely converted into a liquid.

Let's break down the answer into headings and bullet points for a clear explanation:

Definition of Melting Point:
- The melting point of a substance is the constant temperature at atmospheric pressure when the solid phase changes into the liquid phase.
- It is a characteristic property of a substance and remains the same for a particular substance regardless of the sample size.

Understanding the Melting Process:
- When a solid substance is heated, the kinetic energy of its particles increases.
- As the temperature rises, the particles gain enough energy to overcome the attractive forces holding them in a fixed arrangement, and the solid starts to break down.
- At the melting point, the solid and liquid phases coexist, with particles transitioning between the two states.
- The melting point is the temperature at which the solid phase is completely converted into the liquid phase.

Importance of Melting Point:
- The melting point is used to identify and characterize substances.
- It can help determine the purity of a substance, as impurities can cause the melting point to decrease or broaden.
- Different substances have different melting points, making it a useful property for distinguishing between them.

Factors Affecting Melting Point:
- The strength of intermolecular forces: Substances with stronger intermolecular forces tend to have higher melting points.
- Molecular size and shape: Larger molecules with complex structures often have higher melting points.
- Presence of impurities: Impurities can lower the melting point and broaden the melting range.

In conclusion, the melting point of a substance is the temperature at which the solid phase is completely converted into the liquid phase at atmospheric pressure. It is an important characteristic property that can be used to identify and characterize substances. At the melting point, both the solid and liquid phases coexist, and the substance transitions from a solid to a liquid state.