Explanation:

Introduction:
Fins are used in various heat transfer applications to increase the heat transfer from the surface. They are extended surfaces that are attached to the main surface to enhance the heat transfer rate. Fins are commonly used in heat exchangers, radiators, and other cooling systems.

Increasing the effective surface area:
The correct answer to the given question is option 'B', which states that fins are used to increase the effective surface area. This is the primary purpose of using fins. By attaching fins to the surface, the surface area available for heat transfer increases significantly. The increased surface area allows for more contact between the surface and the surrounding fluid, improving the heat transfer rate.

Heat transfer mechanism:
Heat transfer occurs through conduction, convection, and radiation. In the case of fins, conduction is the main heat transfer mechanism. The heat is conducted from the base surface to the fin, and then it is convected to the surrounding fluid. The extended surface area of the fin allows for more conduction and convection, resulting in increased heat transfer.

A comparison of the options:
Let's analyze the other options mentioned in the question and understand why they are not correct:

a) Increasing the temperature difference: Increasing the temperature difference between the surface and the surrounding fluid does not directly affect the heat transfer rate. The temperature difference is a driving force for heat transfer, but it does not increase the heat transfer rate itself.

c) Increasing the convective heat transfer coefficient: The convective heat transfer coefficient depends on the fluid properties, flow conditions, and surface characteristics. While fins can indirectly affect the convective heat transfer coefficient by increasing the flow velocity or turbulence around the surface, the primary purpose of fins is to increase the surface area.

d) None of the above: This option is incorrect because fins do increase the heat transfer rate by increasing the effective surface area.

Conclusion:
In conclusion, fins are used to increase the heat transfer from the surface by increasing the effective surface area. This allows for more contact between the surface and the surrounding fluid, enhancing the heat transfer rate. The other options mentioned in the question are not correct because they do not accurately describe the purpose of using fins.