The best side slope for most economical trapezoidal section, is

ANS:C - 60°

A 60° slope angle represents a steep slope configuration, which is generally not the most economical choice for a trapezoidal section in most engineering applications. However, there may be specific situations where a 60° slope angle could be considered or required:

1. Stability Requirements: In certain geological or soil conditions where stability is a primary concern, a steeper slope angle such as 60° may be necessary to prevent slope failure or instability. For example, in rocky or highly compacted soils, a steeper slope angle could provide better resistance against erosion and slope instability.
2. Space Limitations: In urban areas or constrained construction sites where space is limited, a 60° slope angle might be chosen to maximize land usage while still providing adequate drainage or slope protection. Steeper slopes allow for narrower sections, which can be advantageous in limited space environments.
3. Design Considerations: In some hydraulic or environmental engineering projects, specific design considerations or regulatory requirements may dictate the use of steeper slope angles. For example, in stormwater management systems or erosion control structures, steeper slopes might be required to meet design criteria or regulatory standards.

While a 60° slope angle may offer benefits in certain situations, it is generally less economical compared to shallower slope angles such as 45° or 30°. Steeper slopes require more excavation, grading, and stabilization measures, which can increase construction costs and complexity. Overall, while a 60° slope angle may have niche applications where its advantages outweigh the drawbacks, it is not typically the most economical choice for trapezoidal sections in general engineering practice.