The frontal or the cyclonic rainfall occurs along the front. The front refers to an area where the cold polar air and the warm tropical air collide and do not mix. There develops a boundary called front. This is usually seen in the coastal areas. The condensation of the air along the front forms the clouds and precipitation occurs, called the frontal rain. When the warm air replaces the cold air, it is called warm front and it causes less intense rainfall over a large area. When the cold air replaces the warm air, it is called the cold front and it causes intense rainfall over a small area.
Hence, the correct answer is 'Boundary of warm tropical air and cold polar air'