Right option is (b) 0.74d ≤ c ≤ d
To explain I would say: For serviceability criteria, when transverse and longitudinal stiffeners are provided at one level only, at 0.2d from compression flange d/tw ≤ 250 εw for 2.4d ≥c ≥ d, c/tw ≤ 250 εw for 0.74d ≤ c ≤ d and d/tw ≤ 340 εw for c < 0.74d, where c is clear distance between stiffeners and d is depth of web, εw = √(250/fyw), fyw is yield stress of web.