Which of the following types of cooling is employed in small and medium distribution transformers?

: In small and medium distribution transformers, oil-filled self-cooling (also known as ONAN – Oil Natural Air Natural) is the most common type of cooling method. 

• The transformer is filled with transformer oil, which serves two purposes: insulating the internal components and acting as a coolant. The oil absorbs heat generated in the core and windings of the transformer.
• The heat is then transferred to the tank walls, where it dissipates into the surrounding air. This process is passive and does not require any external cooling equipment.
• The term "self-cooled" refers to the fact that the cooling relies on natural circulation of oil and air (no external pumps or fans are needed in this method).
  
  Additional Information 
• Water filled oil cooled — This is not a typical cooling method used in transformers. Transformers do not use water as the primary coolant inside.
• Oil filled water cooled — This method is sometimes used for large power transformers, where water is circulated around a cooling system to enhance heat dissipation, but it is not typical for small and medium distribution transformers. 
• Water filled self-cooled — Water is not used as the primary coolant in transformer design, as oil provides both insulation and cooling properties.