It is 81, because it has more than two factors (although they are all 3's). The other numbers are prime, because their only factors are 1 and themselves.
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Both 41 and 47 are prime numbers.
All of them except for 31, 37, 41, 43 and 47 which are prime numbers
93 is composite.
prime: 31, 37, 41, 43, 47 composite: the others
93674731 = 19*1481*3329 and so it is a composite number.