The correct answer is "the foot he is not standing on".
Since he could be standing on either foot in any of the positions you list, the free foot is not part of the description of the position.
I note someone said that the answer should be the inside foot. This is often, but not always true. In dances with rise and fall it's most common for the falling step to occur on the outside foot, and a new figure to then begin with a rising step on the inside foot. However, in tango, the majority of promenade figures begin (somewhat arbitrarily, but by convention) with the step of the outside foot.