Neutral axis depth for a singly reinforced beam is the axis at which the stresses are zero and it is situated at the centre of gravity of the section. The depth of neutral axis for a singly reinforced beam is calculated by equilibrium of tensile and compressive forces as shown in figure below:
Where, b = Width of section d = Effective depth of beam Ast= Area of steel reinforcement xu = Depth of neutral axis C = Total compression T = Total tension Total tension (T) = 0.87fy.Ast Total compression (C)=0.36fckxu.b at 0.42xu from the top extreme fibre (as per IS 456:2000) For equilibrium of forces Total Tension = Total compression 0.87fy.Ast= 0.36 fckxu.b xu=0.87fyAst∕0.36fckbd Hence option(a) is correct