Correct answer is (b) (kπ^2E)/ [12(1-μ^2)(b/t)^2].
To explain: The critical stress of infinite plate having width b and thickness t loaded by compressive forces acting on simply supported sides is given by
fcr = (kπ^2E)/ [12(1-μ^2)(b/t)^2],
where μ is Poisson’s ratio of material, b/t is width-to-thickness ratio of plate, k is buckling coefficient and E is Young’s modulus of rigidity of material. The value of coefficient k depends on constraints along non-loaded edges of plate.