In solid mechanics, which option is not a characteristic of a plane stress problem in the XYZ Cartesian system?

(a) One dimension is very small compared to the other two dimensions

(b) All external loads are coplanar

(c) Strain along any one direction is zero

(d) Stress along any one direction is zero

I had been asked this question in unit test.

My doubt is from Plane Elasticity in chapter Plane Elasticity of Finite Element Method