Oil acts as a best coolant material, for transformer cooling. Due to its high efficiency as a coolant it is most widely used in transformers. Not only same but Oil with suitable properties can be used for various power transformers according to their ratings. OPTION (A) IS CORRECT.

Introduction:
Transformers are electrical devices that transfer electrical energy between two or more circuits through electromagnetic induction. They are commonly used in various applications ranging from power distribution to voltage regulation. One important aspect of transformer design is the cooling method employed, which helps dissipate the heat generated during operation. For installations rated at less than 5 kVA, air cooling is generally used.

Explanation:
Air cooling is the most common method used for transformers with power ratings below 5 kVA. This method involves using natural or forced air circulation to cool the transformer. The primary reason for using air cooling in such installations is the lower heat dissipation compared to larger transformers.

Advantages of Air Cooling:
- Cost-effective: Air cooling is a relatively simple and cost-effective cooling method compared to other alternatives.
- Convenient: Since air is readily available, there is no need for additional cooling mediums such as oil or water.
- Maintenance: Air-cooled transformers generally require less maintenance compared to oil or water-cooled transformers.
- Compact design: Air cooling allows for a more compact transformer design since there is no need for additional cooling equipment or systems.

Methods of Air Cooling:
There are two main methods of air cooling used in transformers:

1. Natural Air Cooling:
- In this method, the transformer is designed with sufficient surface area to dissipate heat to the surrounding air.
- Natural convection, which is the flow of air due to temperature differences, is relied upon for cooling.
- The transformer may be equipped with cooling fins or radiators to increase the surface area and enhance heat dissipation.
- This method is suitable for smaller transformers with lower heat dissipation requirements.

2. Forced Air Cooling:
- In forced air cooling, a fan or blower is added to the transformer to enhance airflow.
- The fan circulates the air across the transformer's surface, increasing the heat transfer rate.
- This method is suitable for transformers that generate higher amounts of heat and require additional cooling capacity.

Conclusion:
In summary, transformers rated at less than 5 kVA are generally cooled using air cooling methods. This involves either natural convection or forced air circulation to dissipate the heat generated during operation. Air cooling is preferred for smaller transformers due to its cost-effectiveness, convenience, and ease of maintenance. By utilizing natural or forced air cooling techniques, transformers can operate efficiently and reliably in various applications with lower power ratings.