Hexagonal metals with c/a ratio less than ideal(1.672) shall exhibit twinning 1.When tensile load is applied along the basal plane2.When compressive load is applied along the basal plane3.When tensile load is applied along the prismatic plane4.None of the above

when twinning will happen? for that we need to restrict the slip process. if c/a ratio is less than the ideal ratio slip will occurs on prismatic plane. now to promote twinning we need to restrict the slip on prismatic plane. so, how to do that? here is the schmid factor comes into picture. If schmid factor is zero slip will not occur on that plane. when schmid factor will zero? when your applied axis is perpendicular or parallel to that plane normal. so, the answer is we need apply tensile load along the prismatic plane.