The correct answer is: 0.85 d.
The stress block is a theoretical model used to calculate the stresses in the concrete and steel of a reinforced concrete beam. The depth of the stress block is a measure of how much the concrete is compressed under the load. The maximum depth of the stress block is limited by the tensile strength of the concrete. If the stress block is too deep, the concrete will crack.
The ISI recommendations are a set of guidelines for the design of reinforced concrete structures in India. The ISI recommendations specify that the maximum depth of the stress block for a balanced section of a beam of effective depth d is 0.85 d. This means that the concrete in a balanced section will be compressed to a stress of 0.85 f’c, where f’c is the compressive strength of the concrete.
The other options are incorrect because they do not represent the maximum depth of the stress block for a balanced section of a beam of effective depth d.