3in the wave-mechanical model of the atom, an orbital is defined as
(1) a region of the most probable proton location
(2) a region of the most probable electron location
(3) a circular path traveled by a proton around the nucleus
(4) a circular path traveled by an electron around the nucleus

(2) a region of the most probably electron location. The rest are all false. (1) and (3) are false obviously, because it denotes the location for the proton, which is in the nucleus, and protons don't travel around the nucleus, they are the nucleus, at least a part of it. (4) a circular path traveled by an electron around the nucleus is false because although there are s orbitals which have a spherical shape around the nucleus in its electron shell, it does not mean it travels circularly around the nucleus, nor does it mean it is the only type of orbital shape.

Answer is to avoid experimental error