Answer should be B.
A) Many impulsive adults are not unusually sensitive to dopamine.(The argument has not concluded anything about Adults.)
B) It is not possible to reliably distinguish impulsive behavior from other behavior.(Correct). If it is not possible to distinguish between impulsive and other behavior,The scientist's whole argument will come to an end as the domain itself will not be available to do the experiment)
C) Children are often described by adults as engaging in thrill-seeking behvaior simply because they act impulsively.(Argument doesn't have to do anything with adults' description)
D) Many people exhibit behavioral tendencies as adults that they did not exhibit as children.( Nothing to do with argument)
E) The gene variant studied by the scientist is correlated with other types of behavior in addition to thrill-seeking behavior.(This also has no effect on the argument)
Consider KUDOS if You find it good