What is the correct definition of "spanwise" and "chordwise" with a swept wing?
b) is a correct description of both .
&quot;Spanwise&quot; is perpendicular to the aircraft's longitudinal axis because it does not relate to the width of the wings, but to the span of the aircraft. which includes both the wings and the fuselage. Consider that the wings of most aircraft and all birds, don't really have a clear &quot;root&quot; where  the wing ends and the torso/fuselage begins.
&quot;Chordwise&quot; is parallel to the aircraft's longitudinal axis because that is the way the air flows. Any other angle would not make sense. It might be of interest on some specific airplanes, but otherwise the only thing that matters is the general airflow, which is parallel to the aircraft's longitudinal axis for cruise flight in most any aircraft.
Swept wings, be they aiming forward or aiming back, have a characteristic that can't be defined by either span or chord. The same goes for delta wings for which every chord over the span of the wing is different.