What the F*** has it got to do with the question?
I suspect this has been answered, but...
Fully powered controls = irreversible = neutral most likely defined by position. Therefore tab effectively acts as an elevator in it's own right - assuming nothing is done with the elevator.
Unpowered or power-assisted controls = reversible = neutral most likely defined as zero moment acting. Therefore tab acts to deflect the (much larger) elevator and thus works in the opposite sense to an elevator on it's own.
Which is what the F**** it has to do with the question.
Dick, I agree with the first part of your last statement, but not the second. If the elevator is irreversible, then most-likely it is used to set trim, so it is only in the neutral position by luck - it is the control used to trim the aircraft. Even in the case of the 737 as described by Mono, the trim tab is probably only being used to trim out actuator forces and thus reduce stresses on the system, rather than to trim the aircraft - I'm guessing that it's probably mechanically connected as a pro-balance tab?
Which, as bookworm suggests, seems again to come down to how the neutral position is defined. Is it a fixed angle relative to the fuselage or mainplane, or the position where pitching moments acting about the elevator hinge are neutral, or some moving position depending upon mode of flight / CG / whatever?
This does increasingly sound suspiciously like one of those irritating exam questions where an examiner is much more interested in the candidates grasp of his particular slant on terminology than a genuine understanding of the subject.
G