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Option 1: This statement is incorrect. Diamond is not an allotrope of silicon. It is actually an allotrope of carbon. Silicon has its own allotrope called silicon dioxide.
Option 2: This statement is correct. Diamond is indeed a bad conductor of heat and electricity. This is because its crystal structure does not allow the movement of electrons easily.
Option 3: This statement is correct. Diamond is known to be the hardest substance. It has a unique crystal lattice structure that makes it extremely hard and resistant to scratching.
Option 4: This statement is incorrect. Diamond does not burn to produce carbon dioxide. It is actually a very stable substance and does not react easily with other elements.
Based on the analysis above, the correct answer is option 2: 2, 3, and 4 only.