RTFQ guys,
The question asks ABOUT which does the autopilot control not what does it change to effect the control.
An autopilot (full authority) controls about 3 axis, vertical (yaw), longitudinal (roll) and lateral (pitch). The AC is a function of pitch only so is not the right answer, and, as has been stated moves about so would be rather difficult to use as a reference point. The three axis converge at a point which would equate (roughly) to the a/c C of G (which also moves but to a much lesser degree) so I think that is the answer they are looking for.
As an aside, Qavion, it does not matter where the static port is located, the pressure at the sensing device (ADC or altimeter) is what is measured. If the static port were on the top of the fin (say 60 feet agl) and the ADC in the E&E bay (say 10 ft agl) then for a given local pressure the altimeter would read 10 ft.
[ 22 February 2002: Message edited by: mono ]</p>