Correct option is (b) regime width

To elaborate: For a purely alluvial stream having a span length (L) equal to regime width (W), Lacey’s regime scour depth is given by R’ = 0.473 (Q/f)^1/3.

Silt factor (f) is obtained by the equation f = 1.76 d^1/2 where d is the average size of particle in mm.