IN WHICH OF THE FOLLOWING CASES BREAKING OF COVALENT BOND TAKES PLACE
1. BOILING OF H2O
2. MELTING OF KCN
3. BOILING OF CF4
4. MELTING OF SiO2

Dear User,
The answer is 3. Boiling of CF​4.
This is because CF4 is thermally unstable and decomposes to give Carbonyl fluoride and Carbon monoxide. 
             CF4                     COF2            +        COTetrafluoromethane   Carbonyl fluoride  Cabon monoxide

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3. as it catches fire before boiling point and decomposes... hence breaking of covalant bonds take place!
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