ANS:D - all the above.

1. Balanced Section: This term refers to a critical section in a reinforced concrete beam where the concrete in compression and the steel reinforcement in tension both reach their permissible or allowable stresses simultaneously. At this section, the beam is utilized to its fullest capacity, ensuring an efficient distribution of stresses between the concrete and the steel reinforcement. The design of a beam to achieve a balanced section is crucial for optimizing material usage while maintaining structural integrity.
2. Economic Section: An economic section is one that provides the most efficient use of materials while satisfying the structural requirements. In the context of reinforced concrete beams, an economic section typically refers to a section where the beam's dimensions and reinforcement layout are optimized to minimize construction costs without compromising strength, durability, or safety. Achieving an economic section involves balancing factors such as material costs, construction complexity, and structural performance.
3. Critical Section: The critical section of a reinforced concrete beam refers to a location along its length where the internal stresses reach maximum values. This section is crucial for analyzing the structural behavior of the beam and ensuring that it can withstand the applied loads without failure. The critical section may vary depending on the type of loading, support conditions, and beam geometry. For example, in bending, the critical section is typically located at the supports or regions of maximum bending moment.

In summary, the terms "balanced section," "economic section," and "critical section" are all important concepts in the design and analysis of reinforced concrete beams. While the balanced section ensures optimal utilization of materials by reaching permissible stresses simultaneously in concrete and steel, the economic section aims to minimize construction costs while maintaining structural performance. Meanwhile, the critical section is crucial for understanding the maximum stresses and determining the beam's overall structural behavior.