"The police busted a gang specialising in duping telebanking customers and arrested 15 persons."

The sentence is discussing a police operation where they apprehended a gang involved in fraudulent activities with telebanking customers. The first blank requires a word that describes how the police acted towards the gang. "Busted" is the best choice here, as it refers to a successful arrest of a criminal or a gang involved in illegal activities.

The second blank requires a word that describes what the gang was doing. "Specialising" is the best choice here, as it indicates that the gang was highly skilled and experienced in carrying out the fraudulent activities related to telebanking customers. The word "specialising" suggests that the gang was involved in this activity for a long time and had developed expertise in this area.