In a balanced reinforced concrete (R.C.C.) rectangular beam section (neutral axis within the section), the design moment of resistance is governed by the compatible stresses developed in which material(s)? Choose the most appropriate basis used in limit state/working stress analysis.

Introduction:
A balanced R.C.C. section reaches permissible/limit stresses in both concrete (in compression) and steel (in tension) simultaneously. The moment of resistance is then based on the combined stress resultants of both materials.

Given Data / Assumptions:

• Balanced section: steel just reaches its design stress at the same time as concrete attains its allowable/limit compressive stress.
• Plane sections remain plane; perfect bond between steel and concrete.

Concept / Approach:
The internal compressive force in the concrete block and the tensile force in the steel must be equal and form a couple whose arm determines the moment of resistance. Thus both materials’ stresses determine the design capacity.

Step-by-Step Solution:
1) Compute compressive force C in concrete (equivalent stress block).2) Compute tensile force T in steel (A_s * f_s).3) For equilibrium: C = T.4) Moment of resistance M_r = C * z = T * z, where z is the lever arm depending on neutral axis depth.

Verification / Alternative check:
Balanced condition confirmed when both concrete and steel reach their respective design stresses simultaneously.

Why Other Options Are Wrong:
Steel only / Concrete only: ignores the equilibrium couple; capacity depends on both.

Neither: contradicts basic R.C.C. theory of composite action.

Common Pitfalls:
Assuming steel governs always; in a balanced section, both govern. Also confusing under-reinforced/over-reinforced behavior with the balanced case.

Final Answer:
Steel and concrete both