IC Engine:
An IC (Internal Combustion) engine, also known as a reciprocating engine, is a type of heat engine in which the combustion of fuel takes place within the engine cylinder. It is widely used in automobiles, motorcycles, and various other applications.

Definition:
The most appropriate definition of an IC engine is option 'A' - "Product of combustion is used as the working substance." This definition accurately describes the fundamental principle of an IC engine, where the combustion of fuel generates high-pressure gases that act as the working fluid to produce the desired mechanical output.

Explanation:
To better understand why option 'A' is the correct definition, let's explore each of the given options:

a) Product of combustion is used as the working substance:
This option highlights the key characteristic of an IC engine, where the fuel undergoes combustion inside the cylinder, resulting in the generation of high-pressure gases. These gases act as the working substance or working fluid, which expands and exerts force on the piston, thereby converting the chemical energy of the fuel into mechanical work.

b) Combustion of fuel takes place inside the cylinder:
While combustion indeed takes place inside the cylinder of an IC engine, this option does not fully capture the essence of an IC engine. It fails to emphasize that the combustion process produces high-pressure gases, which are crucial for the engine's operation.

c) Petrol or diesel is used as fuel:
While it is true that petrol or diesel is commonly used as fuel in IC engines, this option does not encompass the entire concept of an IC engine. It focuses solely on the type of fuel used, rather than the fundamental principle of using the product of combustion as the working substance.

d) Works on diesel or otto cycle:
This option refers to two different thermodynamic cycles commonly used in IC engines - the diesel cycle and the Otto cycle. However, it does not encompass the comprehensive definition of an IC engine. IC engines can operate on various cycles, including the diesel and Otto cycles, while still adhering to the principle that the product of combustion serves as the working substance.

In conclusion, option 'A' provides the most appropriate definition of an IC engine as it emphasizes the utilization of the product of combustion as the working substance. This definition encompasses the core principle of an IC engine, where the conversion of chemical energy into mechanical work occurs through the expansion of high-pressure gases generated by the combustion process.