Each can be selected for a reason of its own. The "correct" answer usually involves the "simplest" rule. Here are the possible selections and the corresponding rule(s). Of these, my choice might be 128.
10 - has fewer than 3 factors (=2*5)
30 - has a factor of 3 (=2*3*5)
68 - has a factor of 17, or, has factors that differ by more than 3 (=2*2*17)
128 - has no odd factors, or, only has one prime factor (raised to a power), or, is not a 2-digit number (=2^7)
10 - has fewer than 3 factors (=2*5)
30 - has a factor of 3 (=2*3*5)
68 - has a factor of 17, or, has factors that differ by more than 3 (=2*2*17)
128 - has no odd factors, or, only has one prime factor (raised to a power), or, is not a 2-digit number (=2^7)
