Definition : If sum of all factors is twice the number, then the number is a perfect number
Evaluating the options
I) 6 has
4 factors(1,2,3 and 6) - The sum of the factors is 12(twice the number 6)
Hence, this number is a perfect number,
II) 28 has
6 factors(1,2,4,7,14,28) - The sum of the factors is 56(twice the number 28)
Hence, this number is a perfect number
III) 36 has
9 factors(1,2,3,4,6,9,12,18,36) - The sum of the factors is 91(which is not twice the number 36)
Hence, this number is not a perfect number
I and II only are the perfect numbers, and
Option D is the correct answer
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