The correct answer is Option 4: Nitroglycerine.

Nitroglycerine is a highly explosive compound and is commonly used as an explosive material. It is an oily, colorless, and highly volatile liquid. Nitroglycerine is composed of carbon, hydrogen, nitrogen, and oxygen atoms arranged in a specific molecular structure.

Here are some detailed reasons why Nitroglycerine is used as an explosive:

1. Chemical Composition: Nitroglycerine has a chemical formula of C3H5N3O9, indicating the presence of nitrogen, carbon, hydrogen, and oxygen atoms. The arrangement of these atoms gives Nitroglycerine its explosive properties. It contains a high amount of oxygen, which is crucial for the combustion process and release of energy.

2. Sensitivity to Shock: Nitroglycerine is highly sensitive to shock, meaning that it can easily be initiated to explode by a small amount of energy. This sensitivity is due to the presence of nitro groups (-NO2) in the molecular structure. Nitro groups contain a significant amount of potential energy, and when subjected to shock or heat, they release this energy rapidly, leading to an explosive reaction.

3. High Energy Content: Nitroglycerine has a very high energy content, making it a powerful explosive. When detonated, it releases a large amount of energy in the form of heat, shockwaves, and expanding gases. This energy release can cause significant damage to structures and materials in its vicinity.

4. Industrial and Military Uses: Nitroglycerine is widely used in both industrial and military applications. It is a key ingredient in the production of dynamite, which is a commonly used explosive in construction, mining, and demolition activities. Nitroglycerine-based explosives are also used in military applications such as ammunition and warheads.

It is important to note that while Nitroglycerine itself is highly explosive, it is often stabilized or combined with other materials to form safer and more manageable explosive compounds. For example, dynamite is a mixture of Nitroglycerine and an absorbent material like diatomaceous earth or sawdust, which reduces its sensitivity to shock and makes it easier to handle.

In contrast, the other options provided are not primarily known for their explosive properties:

- Phosphorus Trichloride (Option 1): While it is a reactive chemical compound, it is not commonly used as an explosive.

- Mercuric Oxide (Option 2): It is a toxic compound but not known for its explosive properties. It is more commonly used in laboratory and medical applications.

- Graphite (Option 3): Graphite is a form of carbon that is known for its use as a lubricant, in electrodes, and in various other applications. It is not explosive.

Therefore, based on the properties and common usage, Nitroglycerine (Option 4) is the correct answer for a substance used as an explosive.