Working Stress Method
The working stress method is a design approach used in structural engineering to calculate the dimensions of various structural elements. It involves determining the permissible stress for different materials and using this stress limit to design the dimensions of the structural elements.

Permissible Stress in Steel in Tension
The given question states that the permissible stress in steel in tension is 140 N/mm^2. This means that the maximum stress that can be applied to the steel material under tension is 140 N/mm^2.

Depth of Neutral Balanced Section
The depth of the neutral axis is an important parameter in the design of structural elements. It is the location where the bending stress is zero, and the entire section is in equilibrium. In reinforced concrete beams, the neutral axis separates the compression zone from the tension zone.

Calculation of Depth of Neutral Balanced Section
According to the working stress method, the depth of the neutral balanced section can be calculated as a fraction of the total depth (d) of the section.

The correct answer option 'B' states that the depth of the neutral balanced section is 0.40d. This means that the depth of the neutral axis is 40% of the total depth of the section.

Explanation of the Correct Answer
The depth of the neutral axis in a reinforced concrete beam is determined by the ratio of the permissible stress in steel to the permissible stress in concrete.

In the working stress method, the permissible stress in steel is considered before the permissible stress in concrete. Therefore, the depth of the neutral balanced section is influenced only by the permissible stress in steel and not by the grade of concrete.

Since the given question mentions the permissible stress in steel in tension, the depth of the neutral balanced section is solely dependent on the steel properties and is not affected by the grade of concrete used.

Hence, the correct answer is option 'B' - 0.40d.

C