Right choice is (b) σyx≠0

To explain I would say: A state of plane stress in XYZ Cartesian system is defined as one in which the following stress field exists:

σxz=σyz=σzz=0, σxx(x,y), σxy=σxy(x,y) and σyy=σyy(x,y).

Thus, σxx , σxy and σyy are non-zero stresses. Such a problem in three dimensions can be dealt with as a two-dimensional (plane) problem.